A plurality of parts that constitute an intake passage connected with an intake port of an engine include a constriction part that has a protrusion formed to protrude on an inner circumferential surface side of the intake passage. The constriction part includes cylindrical connections disposed on both sides of a constriction path in which the protrusion is formed. The constriction path has an outer circumferential surface recessed inwardly in a radial direction so as to follow a shape of the protrusion. A portion recessed most of the recessed outer circumferential surface is disposed inside inner circumferential surfaces of the connections in the radial direction. The constriction path has a uniform wall thickness. The constriction part is connected upstream of the throttle body.