Service on a defendant in Japan goes through Japan's Central Authority under the Hague Service Convention, the Ministry of Foreign Affairs in Tokyo. Japan objected to service by judicial officers and other competent persons (Article 10(b) and (c)) when it joined, and on December 21, 2018 it also objected to service by mail (Article 10(a)) and by foreign consular officers (Article 8). Mailing a summons to a Japanese defendant is no longer a valid option.
The Ministry of Foreign Affairs forwards each compliant request to the competent Japanese district court, which serves the defendant under Japanese procedure and completes the certificate. The process is orderly, but it is formal: the package has to meet the Ministry's requirements, including a complete Japanese translation, before anything moves.