A reciprocating body (2) has valve element (6) including deflecting surface (10a), valve shaft (7) guided by guide cylinder (4), and packing (8) having an annular shape circularly brought into close contact with valve seat (12a) in a state where the reciprocating body (2) is in a closed position. The valve element (6) includes large diameter portion (9a), and small diameter portion (9b) and deflecting portion (10). The large diameter portion (9a) has flange surface (9d) supporting the packing (8) in a state where the packing (8) is exposed to the primary flow passage side. The flange surface (9d) is formed such that the flange surface (9d) is allowed to come into contact with the valve seat (12a) via the packing (8). Recess (9c) is formed on the outer periphery of the small diameter portion (9b), and a portion of the packing (8) is fitted in the recess (9c).