Abstract: A pressure-resistant explosion-proof electric motor (44) is surrounded by a double casing comprised of inner and outer cases with a motor-cooling oil passage (54) formed therebetween. A flange (26) is detachably fitted to the top surface of a tank to hold the electric motor (44) in the tank. A downwardly-extending pipe (28) is at an upper end fixed to the flange and at a lower end connected through a terminal box (30) to the electric motor (44). A centrifugal pump (67) is fixed to the underside of the electric motor (44) to have its rotary shaft connected to the output shaft (60) of the electric motor (44). A motor cover accommodates therein the electric motor and the terminal box (30) in a watertight manner. A protective pipe (71) extends from the flange (26) to the motor cover (69) and accommodates therein the downwardly-extending pipe (28) in a watertight manner. A cooling oil is circulated through the motor-cooling oil passage (54) to cool the electric motor.