At trial, it's adversarial, in that the defendant has the right to counsel, and both sides get to argue their case to the panel of judges (There is no jury in Japanese law).
Before trial, however, it's an inquisitorial system. After an arrest, the police can hold a defendant for up to 23 days with no access to counsel, no rights against self incrimination, and no limits on the length of time he can be interrogated.
As a result, over 99% of Japanese criminal trials end in a conviction, based on confessions obtained prior to speaking to a defense lawyer. Additionally, while there is "in theory" a presumption of innocence, virtually ALL judges are former prosecuting attorneys, and the testimony of police officers is simply assumed to be true regardless of any other evidence.
Japanese defense lawyers can even be jailed for defaming the police for claiming in court that a police officer has lied in his testimony.