To smoothly realize the anti-lock control, the traction control, and the automatic brake control by arranging a pressure regulating piston in a hydraulic pressure chamber formed on a hydraulic pressure control device body, and controlling the pressure regulating piston making use of a cam drive mechanism.

When a brake pedal is stepped in, the hydraulic pressure generated in a master cylinder 1 is fed in a hydraulic pressure chamber in a brake device, and a brake piston 4 is moved to give the braking force to wheels. When the wheels are locked, and the brake loosening signal is outputted from an ECU 7, a solenoid valve 2 is locked, and a motor 6 is driven, and a cam shaft 15 is rotated through a pinion 24 and a large gear 23. A roller 23 follows cams 22a, 22b while it is rotated, and pressure regulating pistons 20a, 20b are evacuated in piston storage windows 17a, 17b. Because the volume in the hydraulic pressure chamber 12 is increased while a boot 25 is deformed, the braking hydraulic pressure is reduced, and the locking condition of the wheels is eliminated.