A seat block capable of maintaining a well-fitted state of a seat portion in a simple and reliable manner regardless of use conditions. A seat portion base body (43) is provided in a housing (67) provided with a valve passage (66) and allows a piston (28) for opening and closing a valve passage (66) to be pressed on the seat portion base body (43). A spring member (23) applies an elastic force to the seat portion base body (43) such that the elastic mechanism (23) cooperates with the housing (67) to retain the seat portion base body (43).
