To improve productivity in mass production and cooling performance while thermal fatigue damage at a base of a cooling fan is reduced.

The rotor 1 is mounted in a body with a rotating shaft in an eddy-current type decelerator. The cooling fan formed upright around an outer boundary face of a cylindrical part 1a of the rotor 1 is composed of a plurality of pins 11. If necessary, the area of the pin 11 at the connection part of a cylinder 1a may be made larger than the top thereof, a hollow part 11a with a top opening may be formed in the pin 11 or a buried part 11b in the cylinder 1a may be formed at the lower part of the pin 11. Then, the surface area becomes large, and cooling performance is improved. Heat exchange with high efficiency is carried out at the pin edge when the air is sucked by the rotation of the rotor from both edge sides of the cooling fan to the center and passed through the adjoining pins, so the cooling performance of the rotor can be improved. At this time, the temperature gradient of the base of each pin is small and thermal fatigue damage can be reduced. In addition, the pin can be fabricated in mass production, and the productive cost can be reduced.