This invention pertains to a rotor core (6) in which a plurality of core sheets (1) are laminated on one another on a rotor shaft (4). A plurality of slits (3, 7) and a plurality of strips (2) are alternately arranged in a radial direction of each of the core sheets (1) so as to convexly confront a center of each of the core sheets (1) such that an outer peripheral rim (10) is formed between an outer peripheral edge of each of the core sheets (1) and each of opposite ends of each of the slits (3, 7). A stress concentration portion (11) which is provided at a portion (10a) of the outer peripheral rim (10) and has a width (L1) larger than that of the remaining portions of the outer peripheral rim (10).