A power-assisted mechanism having a simple structure which can perform a power-assisted operation can be achieved in a power-assisted on/off valve including a valve stem which opens and closes a flow passage, an operational member which is positioned on an axis of the valve stem and is relatively movable with respect to the valve stem, and a power-assisted mechanism which reduces the speed and power-assists the movement of the operational member, and transfers the movement of the operational member to the stem valve.The power-assisted mechanism is provided with a transmission rod which is positioned on an axis of the operational member, the transmission rod provided with a reduced-diameter portion that reduces the diameter thereof in a direction toward the valve stem; a pressing member which is interconnected with the valve stem; a pair of motion-transmission surfaces that are formed so as to face the pressing member and a housing that movably supports the pressing member, wherein a space between the pair of motion-transmission surfaces increasingly narrows toward the outer periphery thereof; and a plurality of radial-direction movable bodies that are interposed between the pair of motion-transmission surfaces and the reduced-diameter portion of the transmission rod and are in contact therewith.