My country has no seal certificate. How do I prove my signature for a Japanese company?

With a signature certificate or a notarised affidavit of your signature, issued by a notary in your country or by your embassy or consulate in Japan. The Legal Affairs Bureau accepts this in place of a Japanese seal certificate. It must be recent, so arrange it once the documents are final.

Certification and apostille

Source: 1.3 Procedures for registering establishment, Japan External Trade Organization (JETRO)