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.