A check valve is housed in a valve chamber and includes a support portion and a flexible portion. The support portion extends along a principal axis thereof. The flexible portion is supported by the support portion in a manner extending from the principal axis. A diaphragm defines a lower wall of the valve chamber, and includes an inflow port for fluid at a position that is closed with the flexible portion when the flexible portion is displaced. A bottom plate defines an upper wall of the valve chamber, and faces the diaphragm with the flexible portion arranged therebetween. An uneven portion is provided in at least one of mutually facing areas of an upper principal surface, which is one of two principal surfaces of the flexible portion and faces the bottom plate, and a lower principal surface of the bottom plate, the uneven portion being partially uneven.