PURPOSE: To dispense with an expander, lighten and compact a hydraulic unit by providing a feed-back line for feeding back to a reservoir a brake fluid from the reservoir boosted by the use of a hydraulic pump through a relief valve opened when the brake fluid amount in the reservoir is less than a predetermined value.

CONSTITUTION: A fluid line 14 for returning a brake fluid boosted by a hydraulic pump 6 through a relief valve 13 to a reservoir 9 is provided. The relief valve 13 is adapted to be opened by a projection 16 provided in a piston 15 of the reservoir 9 when the brake fluid amount in the reservoir is less than an amount corresponding to 3kg/cm2 for example. Thus, a portion of the brake fluid boosted by the hydraulic pump 6 is returned into the reservoir 9 through the fluid line 14 and the relief valve 13. As a result, the brake fluid to be stored in an expander 10 is stored in the reservoir 9 which is used in common for the expander.