To prevent a passenger from becoming uncomfortable by differing the natural frequency between a rotor bar edge part which is exposed from the end face of a rotor core, a drive shaft side of the part of an end ring connected to it, and an inverse-drive shaft side.

A rotor bar 14 is placed at a groove being provided at the outer-periphery part of a rotor core 2, an edge part 14a at a drive shaft side and an edge part 14b at an inverse-drive shaft side projects and are exposed from the end face of the rotor core 2 and are mounted with a difference in the length of the exposed part and the rotor 14 and an end ring form a cage- shaped closed circuit. Then, by making different the length of the exposed parts of the edge parts 14a and 14b projecting from the end face of the rotor core 2 at both edges of the rotor bar 14 and then connecting end rings 15a and 15b, a different deformation mode results and a natural frequency becomes lower at the drive shaft side 14a where the exposed part is long and becomes higher at the inverse-drive side 14b. As a result, since the vibration amplitude of each mode cannot be superposed, vibration becomes smaller, thus preventing a passenger from becoming uncomfortable.