To begin with, Article 79 of the Japanese Patent Law reads as follows:
Article 79: A person who, without knowledge of the content of an invention claimed in a patent application, made an invention identical to the said invention, or a person who, without knowledge of the content of an invention claimed in a patent application, learned the invention from a person who made an invention identical to the said invention and has been working the invention or preparing for the working of the invention in Japan at the time of the filing of the patent application, shall have a non-exclusive license on the patent right, only to the extent of the invention and the purpose of such business worked or prepared.
1) What constitutes “use” in Japan? Actual use only, or is preparing for use sufficient? Is continuous use required?
As understood from the law, the “use” meaning “working”, includes preparing for use/working.
2) What is the “critical date” of the prior use? I expect the use must occur before the priority date of the patent.
The prior use must occur before the priority date of the patent.
3) What are key exceptions or limitations to prior user rights? In particular, can the prior user expand upon the original scope of the use, and if so, to what extent?
The prior use can expand the original scope of the use as long as the product or method used is within the scope of the claim at issue.
4) Are there any specific or unique evidentiary requirements for proving prior use?
The prior use must be proved by evidence. The evidence is desired to be preserved in an unalterable form. For example, a laboratory notebook which cannot be altered later is desirable. The evidence is desired to be preserved to memorize facts from the completion of invention/know-how up and preparation of business up to working of the invention.
