A hydraulic brake device for a vehicle includes an auxiliary piston having an effective cross-sectional area larger than that of a master piston, and a pressure transmitting chamber provided between the large diameter portion and the master piston. The auxiliary piston is operatively associated with the assisting operation of an assisting device. A valve mechanism hydraulically closes the pressure transmitting chamber when the actuation of the master piston is assisted by the assisting device through the auxiliary piston and establishes hydraulic communication between the pressure transmitting chamber and a reservoir of the master cylinder when the master piston is not actuated by the assisting device.