PURPOSE: To provide a brake device to operate with slip control embodied in a simple configuration by introducing such an arrangement that the capacity of a relief chamber is varied periodically by a control device and that the internal pressure of brake is decremental in the capacity enlarging period and incremental in the reducing period.

CONSTITUTION: When a monitoring circuit for the rotational tendency of each wheel acknowledges that a decompression is needed to preclude a wheel lock, a control circuit 12 senses the moving condition of a piston 8. When the motion is such that the capacity of a relief chamber 7 is enlarging, a direction control valve 11 is changed over into the position II, and when the motion of the piston 8 is reverse, standby is made until the piston 8 moves to the relief chamber capacity increasing direction, and then the direction contrtol valve 11 is changed over. Thereby the capacity of the relief chamber 7 increases, and the pressure liquid in each wheel brake 5 runs off so that the internal pressure of the wheel brake 5 decreases, to cause the wheel to be accelerated again. After acceleration, the valve 11 returns to the position I. Thus a simple configuration is accomplished.