PURPOSE: To enable service in good performance even in hot liquid by furnishing a heat insulating layer between the outer liquid (lift liquid) and the motor outside wall and between the outer liquid and a pipe for coolant, and by filling these with a gas with small coefficient of heat conductivity or, otherwise, making them vacuum.

CONSTITUTION: A heat insulating casing 12 is installed outside a motor casing 14, and the chamber A is put in vacuum or filled with gas to prevent heat conduction with the outer liquid. A heat insulating pipe 13 is installed also outside a pipe for coolant 11. The space B between the insulating pipe 13 for coolant and the pipe 11 for coolant is in communication with the chamber A, and is connected to a separate vacuum pump or a gas supply device. The chambers A and B are so arranged as kept in stabilized condition at all times. An submerged motor 2 can thus be used in hot liquid in order to prevent heat transfer from the hot liquid.