Provided are an automatic transmission abnormality diagnosis apparatus and method for an automatic transmission having: a lock-up mechanism; a shift mechanism; and a control mechanism. The automatic transmission is adapted to operate in a slip state, a full lock-up state or a lock-up release state of the lock-up mechanism. The lock-up mechanism is forcibly shifted from the slip state or from the lock-up release state to the full lock-up state in an operation region where the lock-up mechanism is not normally placed in the full lock-up state, and then whether the forcible shift of the lock-up mechanism has been successfully performed is determined. As such, the region for executing the abnormality diagnosis of the hydraulic control mechanism is extended and thus the frequency of the abnormality diagnosis increases, and therefore abnormalities can be promptly detected.