There is disclosed a check valve for a tap of a gas or a liquefied gas cylinder comprising a valve body member axially movable in a housing bore. The body member has an axial bore opening out through one axial end of the valve body member and being in fluid communication with an annular space surrounding valve body member within the housing bore. The other axial end of the valve body member is closed and carries a primary seal ring gas tightly engaging an inner circumferential surface in the pore in the closed position of the check valve. There is provided a secondary seal ring at the first axial end of the valve body member. The secondary seal ring has an outer sealing surface in gas tight engagement with another circumferential sealing surface in the housing bore and further has an annular contact surface for gas tight engagement by an actuator member urging the valve body member against the force of a spring to the open position in which the primary seal is disengaged from its associated sealing surface of the housing bore.