A hydraulic booster brake system includes a flow priority valve connected to a discharge portion of a hydraulic pump. To the flow priority valve are connected a brake booster and a power steering booster in parallel. The flow priority valve supplies a stipulated flow quantity of a hydraulic fluid in a total flow quantity of the hydraulic fluid discharged by the hydraulic pump to the brake booster by priority. A relief valve is provided between a brake port and a drain port. A valve body of the relief valve can adjust a relief pressure by turning a screw shaft for relief pressure adjustment.