An opening/closing apparatus for a fuel tank of a vehicle includes: a fuel passage forming portion; an insertion-side opening/closing valve mechanism; a fuel tank-side opening/closing valve mechanism; a liquid discharge path; and an opening/closing valve mechanism that opens and closes the liquid discharge port by a valve body disposed in the fuel passage forming portion, and that includes a moving body movable from a first position where a closing force exerts on the valve body to close the liquid discharge port to a second position where the closing force decreases, the moving body positioned at the first position in a state where the vehicle is stopped, and moving from the first position to the second position when receiving an inertial force due to traveling of the vehicle.