To provide a check valve used in a multistage ejector and having high durability for withstanding use for many hours.

This check valve is composed of a metallic base 17 constituting a bulkhead between a suction port of the multistage ejector and a pressure reducing chamber and provided with a communicating hole 17a communicating the suction port with the pressure reducing chamber, a rubber-made valve member 21 having a thick-walled part 21a provided in a central part and thin-walled parts on its both sides, a pin member 22 passing through the overlapped thick-walled part and thin-walled parts of the valve member and provided with large diameter 22b for preventing coming-off in an end part on one side, and a ring member 23 attached to an end part on the other side of the pin member. The valve member 21 is provided on a pressure reducing chamber side for the base 17 and at a position where the thin-walled parts 21c cover the communicating hole 17a of the base. The base 17 and the thick-walled part 21a of the valve member 21 are nipped by a large diameter part 22b of the pin member 22 and the ring member 23.