PURPOSE: To eliminate the sense of physical disorder from the braking feeling of a vehicle having no auxiliary brake by giving the pressure on the input side of an air master by controlling a bypass valve in stepping-on of a brake pedal where the set pressure is lower than that of the differential pressure regulating valve.

CONSTITUTION: A control unit 9 operates an auxiliary brake 10 by the signal to be inputted by a pressure sensor 14 and starts collection of the braking energy, and at the same time, makes a judgement whether the air pressure P1 on the primary side is higher than the pressure P3(P1>P3). If the judgement is yes, then another judgement is made whether the air pressure P2 on the secondary side is higher than the pressure P3 (P2>P3) by the signal to be inputted from the pressure sensor 21. If the judgement is no, the bypass valve 20 is de-energized and switching is executed to the position 20A, the bypass passage 31 is opened and connected to the air passage 32, and the pressure P3 is given to an air master 16.