To fill a helicopter tank almost full while maintaining the function of a vent passage of the helicopter tank in a perfect state.

Regarding the vent valve and vent passage for the helicopter tank, the vent valve is provided with a closing mechanism 15 for putting the tank in the closed state by closing the vent passage 81 when connection between an airframe and the tank is cut off. The closing mechanism 15 is disposed in a valve housing of the vent valve. The closing mechanism 15 is provided with a movable closing member. To attain the above-mentioned purpose, the closing member of the closing mechanism 15 is formed of a valve ball 10 for closing the vent passage 81 according to the liquid level in the tank 4, and the vent passage 81 is connected to a vent passage of another tank through a connecting passage.