PART I. GENERAL PROVISIONS
Chapter I. Scope of Application
Article 5. (Effect of Foreign Judgments)
Even when a final and binding decision has been rendered by a foreign judiciary against the criminal act of a person, it shall not preclude further punishment in Japan with regard to the same act; provided, however, that when the person has already served either the whole or part of the punishment abroad, execution of the punishment shall be mitigated or remitted.
Reduction of national sentence following pre-trial detention - national proceedings
EDIT.