Grace period in Japan is six months.

In the case of an invention which has fallen under any of the items of Article 29(1) due to the behavior of the person having the right to obtain a patent,
such invention shall be deemed not have fallen under any of the items of Article 29(1) for the purposes of Article 29(1) and (2) for the invention claimed in a patent application which has been filed by the said person within six months from the date on which the invention first fell under any of those items. (see Patent Law Article 30 (2))