To provide a check valve, whose valve body is formed of a single elastic member of an umbrella type, easily providing enough sealability and reduced in size and thickness.

This check valve includes: the valve body having a sealing part having a circular flat surface and a shaft part positioned in the center of the sealing part and formed of elastic material; and a partition wall having a valve element support hole, a circular thin stepped part having a circular flat surface round the valve element support hole and smaller in diameter than the sealing part and a fluid circulation hole positioned in the circular thin stepped part. The shaft part of the valve body is supported in the valve element support hole of the partition wall, and the sealing part is elastically deformed to the partition wall side by bringing the back of the sealing part into line contact with the flat surface circular edge of the circular flat surface stepped part of the partition wall at least in the state where the fluid pressure is not applied to the valve body.