To quickly discharge the gas gasified from a handling liquid in a motor chamber to the outside when a pump is stopped by providing a gas vent valve which passes gas from the inside of the motor chamber to the outside and interrupts the flow of the liquid the outside of the motor chamber to the inside.

The outside of a motor chamber 1 is kept at the pressure higher than the pressure inside the motor chamber 1 by the discharge pressure from a pump chamber 2 during the operation of a submerged pump P, and a gas vent valve 4 is closed. The gas vent valve 4 interrupts the flow of a liquid from the outside of the motor chamber 1 to the inside at this time. When the submerged pump P is stopped, the flow of the handling liquid to the inside of the motor chamber 1 is stopped, and a rotor 11 and a stator 12 are not cooled by the flow of the handling liquid and are quickly heated. The handling liquid remaining in the motor chamber 1 is quickly gasified, the pressure in the motor chamber 1 quickly rises, the pressure on the outside of the motor chamber 1 drops, and the gas vent valve 4 is opened. As the gas is released from the gas vent valve 4, the inside of the motor chamber 1 is again filled with the handling liquid.