A rubber (elastic substance) is attached by vulcanization and an annular sealing member 4 is formed in an annular groove 3 having a step and provided in the bottom wall of a body 2 of a valve disc 1. The sealing member 4 has an annular portion 41 and an annular portion 42. Most of the annular portions 41, 42 are located in a deep groove portion 31 and a shallow groove portion 32, respectively, and the volume of the annular portion 41 is larger than the volume of the annular portion 42. Furthermore, an annular protrusion 43, which protrudes downward, is provided in the boundary region between the annular portions 41, 42. An inclined section that is inclined at the prescribed inner angle Q is also formed in the boundary region.