a reservoir;

a fluid pressure generating unit comprising:

a pump for pumping operating fluid out of the reservoir; and

an accumulator for accumulating the pumped out operating fluid within a predetermined pressure range;

a regulator valve for regulating and outputting a fluid pressure supplied from the accumulator in accordance with an operation amount of a brake operation member;

a housing in which the regulator valve is slidably fitted;

a master cylinder piston which is slidably fitted forward of the regulator valve in the housing, wherein

during normal braking operation, the master cylinder piston is moved by virtue of the output fluid pressure outputted from the regulator valve so as to generate a master cylinder pressure, and

when something abnormal occurs in which the fluid pressure supplied from the accumulator to the regulator valve decreases down to or lower than the predetermined range, the master cylinder piston, which is in press contact with an outer end face of the regulator valve, is moved mechanically so as to generate the master cylinder pressure by mechanically moving forward the regulator valve by virtue of a force applied by the brake operation member;

a normally closed opening/closing valve provided along a fluid line communicating between a side of the accumulator and the reservoir by way of the fluid pressure input side and a fluid pressure output side of the regulator valve or a fluid line communicating directly between the accumulator and the reservoir;

a fluid pressure value detecting unit for detecting a value of the fluid pressure supplied from the accumulator to the regulator valve; and

a control unit for controlling the opening and closing of the opening/closing valve based on detection information from the fluid pressure value detecting unit,

wherein the control unit changes over the state of the opening/closing valve from a closed state to an opened state in the event that the control unit determines that the fluid pressure supplied from the accumulator to the regulator valve decreases down to or lower than the predetermined range, based on the detection information from the fluid pressure value detecting unit, whereby any residual pressure remaining in the regulator valve is reduced to facilitate forward mechanical movement of the regulator valve, and to promote manual application of the brakes.