To provide a retarder control device which can decelerate the speed of a car body normally and smoothly, even when a torque converter disturbs the transmission of the engine brake torque resulting from an imperfect operation of a lock-up piston.

The engine rotational frequency NE found from the output of an engine rotation sensor 20, and the turbine rotational frequency NT found from the output of a turbine sensor 27 are compared, and when the engine rotational frequency NE < the turbine rotational frequency NT, even though a lock-up piston 32 is operated in its fastening direction, it is decided that the fastening of the pick-up piston 32 is not perfect, and a braking force is output from a retarder device 13.