A hydraulic braking pressure control system includes a shut-off valve (4) which is provided in an oil passage (58) connecting a brake device with an output port in a master cylinder and which is brought into a closed state by a control circuit when a braking operation is detected by an operation-amount detector, and a locking device (5) for inhibiting the closing of the shut-off valve when the output hydraulic pressure from a hydraulic pressure supply source in equal to or less than a predetermined value. The shut-off valve is opened when there is no braking operation, thereby ensuring that even if a hydraulic pressure is delivered from a hydraulic pressure control valve when there is no braking operation, such hydraulic pressure can be released into a reservoir. In addition, if the hydraulic pressure in the hydraulic pressure supply source is abnormally reduced, that shut-off valve is locked in its opened state by the locking device to ensure that the output hydraulic pressure from the master cylinder can be applied to the brake device.