An input piston connected to a brake pedal and an output piston which is moved forward to increase a hydraulic pressure in a pressure chamber are fitted to each other telescopically, and an engaging spring is provided between the input piston and the pressure chamber. The output piston is moved forward relative to the input piston by a hydraulic pressure in a rear chamber while compressing the engaging spring. Since a set load of the engaging spring is set at a small value, the output piston is moved forward relative to the input piston in an initial period of a braking operation, making it possible to reduce a stroke of the input piston which is required for a hydraulic pressure in brake cylinders to reach a first set pressure.