PURPOSE: To reduce the stepping force of a pedal, by energizing the pedal in direction of reducing its stepping force by means of an assist piston which operates itself upon receiving delivery pressure at the delivery port of a valve body in time of pedal stepping on.

CONSTITUTION: When a brake pedal 13 is stepped on, a piston 25 is set in motion and an exhaust valve seat 38 takes its place to a valve member 34 and afterward, the valve member 34 comes off a feed valve seat 39. And, compressed air in a first input chamber 36 flows into the inside of a first control pressure chamber 27 and further the compressed air flows into one side of brake actuators by way of a first output port and simultaneously flows into a relay chamber 45. At this time, the compressed air in the control pressure chamber 27 also flows into the inside of a pressure air chamber of a stepping force reducing device 20 by way of a pipe 87 whereby an assist piston 82 is driven toward the upper part against a spring 84. Therefore, the pedal 13 receives turning force in the stepping direction via a roller 18, thus stepping force is reduced. In addition, with the compressed air in the relay chamber 45, a piston 47 is operated so that the other side brake system is set in motion in an interlocking manner.