To provide an opening/closing valve reducing a fluid leakage amount without enlarging a size.

In a worm gear 21 and a helical tooth 30b of a gear 30, drive force is transmitted from a motor 20 to the gear 30, but a meshing angle is formed without transmitting drive force from the gear 30 to the DC motor 20. A spring 34 is interposed in a drive force transmitting route from the gear 30 to a shaft 32. Even after a valve member is seated in a valve seat, the DC motor 20 is further rotated in a valve closing direction, the spring 34 is wound up, by elastically deforming the spring 34, its energizing force acts in a direction pressing the valve member including a spool 37 to the valve seat. Accordingly, when an opening/closing valve is closed, leaking of vaporized fuel from a contact part between the valve member and the valve seat can be suppressed. Even when the energizing force of the spring 34 acts in a valve opening direction of the gear 30, by locking meshing of the worm gear 21 and the gear 30, even when carrying of a current in the DC motor 20 is turned off, the opening/closing valve is surely closed.