A valve device includes a tubular member fitted into an inflatable article, a flexible diaphragm having a peripheral region which engages a valve seat of the tubular member through a mounting support and an intermediate surrounding region which has a portion that can be flexed to deflect a corresponding portion of the peripheral region from the valve seat, and a positioning member which is actuated to be displaced between normally-closed and deflating positions so as to bring an engaging end thereof to be proximate to or distal from the valve seat. In the deflating position, the flexed portion is vested with an increment of biasing force that is counteracted by a resisting force at an actuated end so as to keep the peripheral portion in a deflected position for maintaining the valve device in an open state.