To provide an automatic transmission mounting retarder capable of decelerating and stopping a vehicle body by a sensation of normally braking the body by outputting a brake force to the retarder even when a throttle sensor is defective.

An automatic transmission control unit 30 decides the operation or non-operation of a retarder 13 according to the set state of a range by a select lever 19, the presence or absence of depressing of a brake pedal 18, and the operating state of an accelerator pedal 17. Even if the idle state cannot be identified from the output of a throttle sensor 23 in a first speed range or a second speed range requiring for an engine brake, the retarder is operated. Since the operating amount of the retarder is corrected according to the vehicle speed, the gross weight of the vehicle body and the inclination of a descent, smooth deceleration and effective stop are realized with the same sensation of braking.