When claiming priority under the Paris Convention, priority documents must be submitted within 3 months from the filing date in Japan. (For a Hague application designating Japan, within 3 months from the date of international publication). If the priority application is filed in a country / region where the Digital Access Service (DAS) of the WIPO is available, it is also possible to file the access code instead of submitting a priority document by mail.

If the deadline for submitting priority documents has passed

If the priority document is not submitted within the above period, the JPO will send a reminder notice to the Japanese agent. (For a Hague application designating Japan, directly to the applicant). In this case, the applicant can submit the priority document within 2 months from the dispatch date of the reminder notice from the JPO. No fee or declaration is required.

If the applicant fails to submit a priority document within the above period due to circumstances beyond his/her control, further relief may be granted under prescribed conditions. The requirements for the relief are quite complicated so it is recommended to contact Japanese counsel for more detailed information.

If the priority period has passed

Applicants are entitled to obtain relief if the application is filed in Japan within two months after the expiration of the priority period with the requirements (1) – (3) below. Please note that this remedy is not applicable for Hague applications designating Japan.

(1) Declaration that failure to meet the priority deadline was not intentional
(2) Reasons for Recovery and Documentary Evidence
(3) Restoration Fee (24,500 yen for each design application)

Please also refer to the following link.
JPO Guidebook for Overseas Users: https://www.jpo.go.jp/resources/report/sonota-info/document/panhu/design_right_injapan.pdf (JPO website)