A pump apparatus has a regenerative pump and a motor for driving the regenerative pump. A substantially annular flow passage is formed in the pump housing of the regenerative pump and communicated with a discharge port and a suction port. An impeller is rotatably housed in the pump housing. A plurality of radial vane grooves are formed circumferentially spaced in the outer peripheral portion of each surface of the impeller. A recess is formed in the portion of one of side surfaces of the flow passage aligned axially with the discharge port formed in the other side face of the flow passage.
