PURPOSE: To minimize nonconformity such as deterioration in fuel consumption, exhaust composition, or car operability by diagnosing a fault of lockup mecha nism, based on both values detected by a pressure detecting means and connection/disconnection-indicating-conditions of lock-up mechanism.

CONSTITUTION: If the running condition of a vehicle at present belongs to a lock-up region, a control unit 6 for an automatic transmission, into which signals from respective sensors are inputted, judges whether or not the mechanism is under the lock-up connection command. If it is YES, the unit 6 judges whether or not an output of a pressure sensor (line pressure P) is lower than a criterion value. If it is YES, because it means that the line pressure P was lowered as required, it can be judged that a solenoid valve 55 is working normally. When the line pressure P is higher than the criterion value, because it means that the solenoid valve 55 does not work (go into an open-valve condition) as instructed, it is judged that the solenoid valve 55 is out of order. Then, the unit 6 lights an alarm lamp, so that a driver recognizes the fault to take proper steps.