PURPOSE: To prevent crack between the outermost part of a slot hole and the circumferential profile of a core by composing a laminated core of a low magnetostriction material at the opposite end parts thereof in the axial direction and setting the dimension between the circumferential profile and the outermost part of the slot hole larger as compared with normal electromagnetic steel plate part.

CONSTITUTION: A rotor core 8 comprises laminated cores 12, 13 of low magnetostriction iron plate at the opposite end parts in the axial direction and a normal laminated core 14 at the intermediate part. Dimension A between the peripheral profile 3a of a punched plate 1a constituting the normal laminated core 14 and the outermost part 4a of a slot hole 2a is set as small as possible on the characteristics of motor. Dimension B between the peripheral profile 3b of a punched plate 1b constituting the laminated cores 12, 13 of low magnetostriction material and the outermost part 4b of a slot hole 2b is set larger than the dimension A. The low magnetostriction iron plate is fragile and subjected to crack, but crack can be prevented by setting the width B wide.