PURPOSE: To prevent a driver from feeling a sense of incongruity caused by the change of brake feeling also during exhaust brake releasing in order to prevent any speed change shock.

CONSTITUTION: A control unit 10 normally opens an solenoid valve 29 and operates an operating cylinder 24 by air pressure from an air tank 27 while an exhaust brake switch 30 is turned on, and then it closes the butterfly valve 22 of an exhaust pipe 21 and applies braking force by the exhaust brake operation. When the downshift of an automatic transmission 3 is performed during this period, the solenoid valve 29 is turned off so as to release the exhaust brake during the speed change, while a retarder device 7 is operated according to the exhaust brake releasing time. Thereby, braking force is applied instead of exhaust brake.