When to file provisional patent application

You're right, there is no Japanese analogue to a U.S. provisional patent application. A provisional application establishes a priority date but is not examined and must be followed within one year by a regular nonprovisional application. The reasons to file a provisional application include deferring some costs and/or getting something on file quickly, since a provisional application doesn't have to include all the formalities of a nonprovisional application. It also sometimes makes sense to file a provisional application if the invention is still being refined and has not yet reached its final form. The best time to file a provisional application is before any public disclosure of the invention, to make sure you preserve patent rights in all countries.

You should file a provisional patent application as soon as you have finalized, as best as you can, your invention. You then have 12 months to perfect or convert the provisional patent application into a non-provisional patent application. If you want to get a patent, you must convert the provisional application to a non-provisional patent application because a provisional patent application will never be examined and will therefore never lead to a patent. The provisional application effectively provides you with your earliest filing date for your invention. If you make additional improvements, or develop additional embodiments, you can file additional provisional applications, HOWEVER, your conversion to a non-provisional application must be accomplished within the one year from your first provisional application filing. What we normally do in such cases is simply add the new material to the original provisional application, and then at the end of the one year period, file the last, compiled, provisional application as the non-provisional application. If you also desire to foreign file the invention, you must also file a PCT application within the one year period from the first provisional application filing. If you file the PCT application, you do not necessarily have to file your non-provisional application here in the US at that tine, although you can - you can file the US non-provisional application as a national stage application under PCT at 30 months from the initial provisional application filing.