In a hydraulic braking pressure control system including an auxiliary hydraulic pressure generating unit capable of generating a hydraulic pressure in accordance with the braking operation of a brake pedal, and a hydraulic pressure supply source hydraulic pressure control unit interposed between a hydraulic pressure supply source and a brake device for controlling the hydraulic pressure from the hydraulic pressure supply source in accordance with the braking operation of the brake pedal, the hydraulic pressure chamber in the auxiliary hydraulic pressure generating unit is connected to the supply source hydraulic pressure control unit, so that the output hydraulic pressure therefrom is applied as a control pressure. When the hydraulic pressure from the hydraulic pressure supply source has been abnormally reduced, a braking pressure from the auxiliary hydraulic pressure generating unit is applied to the brake device in response to an opening of the on-off valve. During braking in a condition in which the hydraulic pressure from the hydraulic pressure supply source is normal, the operation of the hydraulic pressure regulating unit avoids the hydraulic pressure from the auxiliary hydraulic pressure generating unit from being applied to the brake device.