A flow direction restriction valve mechanism unit includes a pressure decreasing valve assembly that is located to freely advance or retract in a combination valve cassette attached to an in-outlet secondary side flow path formed on the side of an outlet with respect to an intermediate transmission member in a flow path, and retracts by a pressure of gas; a first coil spring urging the pressure decreasing valve assembly in a retracting direction; a check valve that is located to be freely advance or retract with respect to the pressure decreasing valve assembly, and retracts by a pressure of the gas; and a second coil spring urging the check valve in an advancing direction. A gas induction passage is formed in which the gas flows and acts on the pressure decreasing valve assembly such that the pressure decreasing valve assembly moves in the advancing direction.