Patent Infringement in Japan: How to Check Your Risk and Respond to a Warning Letter

"We received a patent warning letter from a Japanese company." "Amazon Japan removed our listing for patent infringement." "Before we launch in Japan, how do we know our product is safe?"

For overseas manufacturers, exporters and e-commerce sellers, a Japanese patent can stop sales overnight. In this article, Japanese Patent Attorney Toru Kano of BrandAgent explains how patent infringement is judged in Japan, what the risks are, how to check your product, and what to do — and not do — if a warning arrives.

Key points

Infringement is judged by whether your product meets every element of a claim in the Japanese patent.
Even if it does not match word for word, the "doctrine of equivalents" or "indirect infringement" may apply.
Importing and selling in Japan count as working the invention. "We didn't know" is not a defence to an injunction or damages.

What is patent infringement in Japan?

A Japanese patent gives its owner the exclusive right to work the patented invention in Japan. Only the patent owner, or someone licensed by the owner, may lawfully work it. Anyone else who works the invention in Japan as a business without permission infringes the patent.

Patents are territorial. A Chinese or US patent has no effect in Japan, and a Japanese patent has no effect outside Japan. What matters for your Japanese sales is whether a Japanese patent covers your product.

Importing into Japan counts as "working" the invention

Article 2(3) of the Japanese Patent Act defines "working" broadly. For a product invention, it includes producing, using, selling or otherwise transferring, leasing, exporting, importing, and offering for sale (including displaying for sale). For a process invention, it covers using the process and, for a production process, the products made by it.

This means that even if your product is made entirely outside Japan, importing it into Japan or listing it for sale on Amazon Japan or Rakuten can be an act of infringement.

Three requirements for infringement

Basic requirements

No right to work the invention: you are not the patent owner and have no licence covering the act.
You "work" the patented invention: your product or process falls within the scope of the patent claims (the "technical scope").
As a business: working the invention purely for personal or household purposes is not infringement.

The scope of a patent is defined by its claims. Whether your product falls within the claims is the starting point of every infringement analysis.

Three types of infringement

1. Literal infringement

Your product meets every element of a claim as written. If the claim consists of elements A, B and C, and your product has A, B and C, it literally infringes. This is the basic form of infringement.

2. Infringement under the doctrine of equivalents

Even if your product does not match the claim word for word, it can still infringe if it is substantially the same invention — for example, A + B' + C, where B has been replaced by an equivalent B'. Otherwise, anyone could avoid a patent by changing a minor detail.

In its judgment of February 24, 1998 (the "Ball Spline" case), the Supreme Court of Japan held that equivalence is established when all five of the following requirements are met.

The five requirements for equivalence (Supreme Court, Feb. 24, 1998)

The differing part is not an essential part of the patented invention.
Even with the replacement, the purpose of the invention is achieved and the same function and effect are obtained.
A person skilled in the art could easily have conceived of the replacement at the time of manufacture.
Your product is not identical to, and could not easily have been inferred from, the prior art at the time of filing.
There are no special circumstances, such as the replaced element having been intentionally excluded from the claims during prosecution.

3. Indirect infringement

Article 101 of the Patent Act treats certain acts that are highly likely to lead to infringement as infringement. Examples include producing or selling, as a business, a part that is used only to produce the patented product, or supplying an item that is indispensable to solving the problem the invention solves, while knowing it will be used to infringe.

For component and OEM suppliers, this is a key point: "we only sell parts" is not always a safe position.

What happens if you infringe a Japanese patent?

Main risks

Injunction: the patent owner can demand that you stop importing and selling, and that infringing products and equipment be destroyed or removed.
Damages: under Article 103, an infringer is presumed to have been negligent, so it is relatively easy for the patent owner to claim damages.
Restoration of reputation: the court may order measures such as a published apology.
Criminal penalties (intentional infringement): imprisonment of up to 10 years and/or a fine of up to JPY 10 million (Article 196). Where the infringement is committed in a company's business, the company itself can be fined up to JPY 300 million (Article 201).
Marketplace action: in practice, a patent owner may also file a complaint with Amazon Japan or other platforms, leading to listing removal.

Apart from criminal liability, not knowing about the patent does not protect you from an injunction or a damages claim. That is why checking before you launch matters.

How to check whether your product infringes

Find the relevant patents: Japanese applications are published 18 months after filing, and granted patents are published in the patent gazette. You can search them free of charge on J-PlatPat, the JPO's database. Most documents are in Japanese, so machine translation alone can be misleading.
Determine the technical scope: read the claims carefully, referring to the description and drawings where needed, to understand exactly what the patent covers.
Compare your product: check whether your product falls within the scope from the viewpoints of literal infringement, equivalents and indirect infringement.

Judging infringement correctly requires careful claim interpretation in Japanese. If you are unsure, ask a Japanese patent attorney before drawing your own conclusion.

How to reduce the risk before launching in Japan

Run a freedom-to-operate search at the product planning stage and again before launch, focusing on Japanese patents.
Monitor new filings in your product field — new applications by competitors may be published after your first search.
Check your supply chain: if you buy products from an OEM manufacturer, ask about their patent position and include IP warranties in your contract.
Have a Japanese patent attorney you can call — when a problem arises, speed matters.

You received a warning letter: what to do first

Initial response to a warning letter

Don't ignore it, and don't admit anything: do not reply hastily. Consult a Japanese patent attorney first.
Check whether you really infringe: confirm that the patent is valid and in force, and compare its claims with your product. Warnings are sometimes based on a misunderstanding.
If infringement is likely, stop: continuing to sell after a warning can be treated as intentional infringement and raise the risk of criminal liability.
Consider invalidating the patent: if you can find prior art published before the filing date, you may be able to invalidate the patent through an invalidation trial at the JPO.

The same applies to an Amazon Japan infringement notice: check which patent is cited, confirm its scope, and decide on a response with professional advice rather than simply deleting the listing or ignoring the notice.

Summary

Only Japanese patents matter for sales in Japan, and importing or selling in Japan counts as working the invention.
Infringement is judged against the claims, including equivalents and indirect infringement.
Risks include injunctions, damages, criminal penalties and marketplace removal.
Search before launch, and if a warning arrives, don't ignore or admit — get advice first.

FAQ

Q. Our product is made in China. Can a Japanese patent still stop us?
A. Yes. Importing into Japan and selling or offering for sale in Japan are acts of "working" the invention, so a Japanese patent can be enforced against those acts regardless of where the product is made.

Q. We changed one part of the design. Are we safe?
A. Not necessarily. If the change is not essential and the product works in substantially the same way, the doctrine of equivalents may apply. A claim-by-claim analysis is needed.

Q. Can we search Japanese patents ourselves?
A. Yes, J-PlatPat is free. However, the documents are mostly in Japanese and claim interpretation is technical, so we recommend having important results reviewed by a Japanese patent attorney.

Q. What if the patent itself seems weak?
A. If prior art existed before the filing date, you may be able to challenge the patent through an invalidation trial at the JPO. This can be an effective response to a warning letter.

Received a Japanese patent warning or want to check your product before launch?
Send us the patent number or a description of your product — a Japanese patent attorney will review your situation.

BrandAgent Patent Office (Kyoto, Japan)
Toru Kano, Japanese Patent Attorney