PURPOSE:To effectively prevent water from leaking, lubricating oil of bearings from decreasing in lubrication by a simple method by impregnating by heating impregnation method rust preventive oil in a gap between laminated core of a rotor and a gap between the coil and a shaft, and forming insulating membranes in the gap between the laminated core and between different metals. CONSTITUTION:An aluminum conductor 3a is cast in the groove 4 of a rotor laminated core 1, in which silicon steel plates 2 are laminated, both ends of the conductors are shortcircuited with cast squirrel cage coil 3 formed of end rings 3b of aluminum, and the core 1 is engaged with a metal rotor shaft 5. A corrosion resistance is formed by dipping the thus composed rotor 6 in rust preventive oil adjusted in viscosity in approx. 50sus heated, for example, to 150 deg.C, and insulating membranes 7 are formed in the gap between the cores 1 and the gap between the core 1 and the coils 3, shaft 5. In this manner, water leakage, or decrease in lubrication of bearings can be effectively prevented.