To provide a check valve of a positive displacement type pump capable of reducing pressure loss of fluid and improving pump efficiency.

This check valve is provided by facing reversely on a fluid feeding-out side and a fluid absorption side of a pump chamber 22b, a valve element 40 is opened and closed by only pressure of fluid flowing in or out of the pump chamber 22b, the valve element 40 has one shaft axis parallel with a flow passage of fluid, and an outer shape when viewed in a plane is formed into a circular shape centered on the shaft axis.