To provide a check valve with a pressure relaxation structure, having reduced cost and size by reducing the number of parts for easy manufacture.

A check valve element 15 is stored in a valve case 12, and a spring 17 is interposed between a cap 14 and the check valve element 15 to hold the check valve element 15 in a normally closed position. At a central portion of the check valve element 15, a pressure release passage 15g is provided for communicating a primary side with a secondary side. An engaging groove portion 15h is formed on the end face on the primary side of the check valve element 15, and the outer peripheral edge of a pressure release valve plate 18 integrally formed of an elastic material is fitted into the engaging groove portion 15h. When pressure on the secondary side is abnormally high, the pressure release valve plate 18 itself is elastically deformed by high pressure operating on a seal face 18b of the pressure release valve plate 18 through the pressure release passage 15g to form a gap between the check valve element 15 and the pressure release valve plate 18. Fluid is moved from the gap via a pressure release hole 18a to the primary side to relax abnormal pressure on the secondary side.