A pressure regulating valve unit 7 is configured to regulate the output hydraulic pressure of a hydraulic pressure generating source 6 through electrical control for application to a boosted hydraulic pressure chamber 9, and a release valve 103 is interposed between the boosted hydraulic pressure chamber 9 and a reservoir R which is adapted to the hydraulic pressure in the boosted hydraulic pressure chamber 9 to the reservoir R side when the pressure regulating valve unit 7 fails.
