To give no unpleasant feeling, even if the brake force of a retarder device and an engine brake is overlapped each other and outputted by dropping the brake force of a retarder device by receiving a measurment signal, when a prescribed time is measured by a time measurement means by receiving the start signal of the retarder device.

A retarder device 21 is arranged in the output shaft side of an automatic transmission 12 and outputs a brake force in response to an oil pressure level supplied from a retarder drive device 22 to a propeller shaft 13. Usually, a speed change control unit 18 decides the level of brake and an output timing by the operation of a retarder switch 23 and the retarder device 21 is operated by the retarder drive device 22. A measurement is started by a timer, when a brake force by the retarder device 21 is outputted and the brake force by the retarder device 21 is dropped by a prescribed value, when the brake force of an engine brake by a down shift after a prescribed time lapse is outputted. Consequently, a total brake force is kept at nearly constant until speed change finish.