A braking fluid pressure control apparatus is provided including a master cylinder driven by force applied to a brake pedal for generating braking hydraulic pressure; an auxiliary power unit sucking fluid from a reservoir and pressurizing the fluid so as to store the fluid as an auxiliary power source; a booster having a boost chamber for generating boost pressure proportional to force applied to the brake pedal in the boost chamber in receipt of pressure supplied from the auxiliary power unit so that the thrust of the master cylinder is increased by the boost pressure generated by the force applied to the brake pedal and the boost pressure is used as braking pressure. A pressurizing unit housing is provided with a large-diameter opening and a small-diameter opening arranged in series. A first piston is provided which is slidably inserted into the small diameter opening and fluid-sealing the small-diameter opening so as to separate a first cylinder chamber and a second cylinder chamber from each other. A second piston slidably is inserted into the large-diameter opening so as to separate the second cylinder chamber and a third cylinder chamber form each other. A spring is provided for urging the second piston toward the second cylinder chamber and for opening and closing a port. The apparatus permits brakes to operate effectively even if a problem exists in the boost pressure line.