To improve sealing ability by preventing dislocation of an O-ring to hold sealing ability of a relief valve, to improve an automatic regeneration function by eliminating a need for repair and confirmation of dislocation of the O-ring, and to improve working efficiency.

This valve mechanism is constituted such that an O-ring 4 is located at the end part of the exhaust pipe 8 of a cryopump 1 and sealing ability of the relief valve 3 arranged at the external part side of the exhaust pipe 8 is held by the O-ring 4. In this case, a fixing ring 10 having an inside diameter smaller than the outside diameter of the O-ring 4 and height lower than the ring size of the O-ring 4 is arranged on the outer peripheral side of the O-ring 4. The O-ring 4 is pressed against the exhaust pipe 8 by the fixed ring 10 for mounting.