A piston member is slidably accommodated in a housing for defining a master pressure chamber. A simulator piston is provided for defining a simulator chamber and moving in response to operation of a brake pedal. The simulator chamber is communicated with an atmospheric pressure chamber when the piston member is placed in an initial position thereof, and the communication between the master chamber and the atmospheric pressure chamber is blocked when the piston member is advanced from the initial position thereof by a first stroke or more. Furthermore, the communication between the simulator chamber and the atmospheric pressure chamber is blocked when the piston member is advanced from the initial position by a second stroke, which is set to be greater than the first stroke, or more. And, a blocking member is provided for restricting the simulator piston to be retracted up to a position thereof which is placed relative to the piston member when the brake pedal is inoperative.