A device for filling and withdrawing gas includes a withdrawing circuit provided with a storage connector that can be connected to a storage container and a withdrawing connector that can be connected to a withdrawing device. The device also includes a filling circuit provided with a filling connector that can be connected to a gas source for filling the container. The filling connector includes a piston internally defining a filling duct and having a drain orifice opening transversely into said filling duct. The piston is movable inside a drain body provided with a drain duct connected to a leakage circuit connected to the outside, between an open position wherein the drain orifice does not coincide with the drain duct and a closed position wherein the drain orifice coincides with the drain duct to establish communication between said filling duct and the leakage circuit.