A first check valve has a narrow flow path portion (68) formed between an arcuate portion (46) of the outer periphery (45) of a support member and a second enlarged-diameter portion (11) of a casing. The narrow flow path portion (first portion of an intermediate flow path) is in fluidic communication with a back-pressure chamber (50) defined in the support member at the rear of the top of a valving element through a communicating passage (67) including a gap (66) between the outer periphery of the valving element and the inner periphery of a large-diameter portion (38) and grooves (27) on the rear surface of the top of the valving element. Thus, a low static pressure of fluid flowing through the narrow flow path portion is introduced into the back-pressure chamber to move the valving element by a large differential pressure during the supply of fluid, thereby allowing the valve opening to become larger than in the conventional apparatus and enabling a reduction of pressure losses. An intermediate chamber has a pressure chamber (130) defined therein by using a rigid partition. A high-pressure chamber (131) of the pressure chamber is in communication with the upstream side of the first check valve, and a low-pressure chamber (132) thereof is in communication with the narrow flow path portion. Thus, a valving element (136) is acted upon by a force larger than the differential pressure between the upstream side and an internal flow path (106) of the intermediate chamber, thereby allowing a relief valve (135) to be surely kept closed.