An inner surface of the panel is formed to satisfy at least one of the following relationships: ZPHLPH0. The brake pedal is coupled through a spring to the spool valve of the proportional pressure control valve having a solenoid for generating a spool-driving force, and the brake pedal force is applied to the spool valve as an external force so that not only in an emergency but in a normal state, pressure is adjusted by the brake pedal force. The first-stage arbiter may operate in a first pipelined stage, and the second-stage arbiter may operate in a second pipelined stage.