A motor includes: a rotating shaft having a key groove formed in the peripheral surface of the rotating shaft; a rotor core having a through hole in which the rotating shaft can be received and a key portion formed on the inner surface of the rotor core defining the through hole, the key portion being fittable in the key groove; and a stress relaxation groove formed at a position adjacent to the key portion in the inner surface of the rotor core defining the through hole, and depressed away from the peripheral surface of the rotating shaft. The inner surface of the through hole is arc-shaped. A bottom surface of the stress relaxation groove, located radially outward in the inner surface of the stress relaxation groove, includes an arc portion centered at the center defining the through hole.