PURPOSE: To surely provide deceleration in accordance with braking torque in a low speed condition by applying a retarder brake position in an opening/ closing valve in the case that a retarder switch is turned on to detect a specified running speed or lower and further to detect a brake pedal in a step-on release condition.

CONSTITUTION: A brake pedal 94a provides a brake release condition detecting means 99 closed in a step-on release condition of the pedal and a speed detecting means 100 closed at a specified running speed or lower of a vehicle. When detected the specified running speed or lower, an opening/closing valve 96 is switched to a position (e). When a position (c) is taken by the second switching valve 32, a pressure of air in a pressure air source 4 is relatively low reduced to be adjusted by one pressure reducing valve 33, to introduce the air into an air chamber of an air booster 93, and in the case of a position (d), the air is introduced into the air chamber at a relatively high set pressure. By this brake fluid, front friction brake devices 91, 92 are braked.