PURPOSE: To provide a bypass valve which is actuated in an interlocking manner with the actuation of a check valve, opened when the water pressure in the down stream abnormally rises, and prevents the water hammer when the non-return valve is actuated.

CONSTITUTION: A bypass passage 12 to be communicated with a downstream port 13 and an upstream port 15 is provided in a valve box 11 of a check valve 1, and a bypass valve 2 to open/close a valve seat 14 of the valve box of the flow passage is mounted. This valve is provided with a valve element to be attached/detached to/from the valve seat of the valve box, and the valve element is energized in the direction to constantly close the valve by an elastic spring 29 from the rear side. When the water pressure in the downstream is abnormally increased, the check valve is closed to prevent the reverse flow to the upstream, and the water pressure presses back the valve element against the energizing force of the elastic spring of the bypass valve to be detached from the valve seat of the valve box, and the downstream is communicated with the upstream in the check valve. The water pressure in the downstream is rapidly released to prevent the occurrence of the water hammer. When the abnormality in the flow passage is converged, the bypass valve is automatically returned to the original condition together with the cheeck valve. The water hammer when the flow passage is in an abnormal condition is prevented.