A rotor, comprising: a rotating shaft with at least one keyway formed at an outer circumferential surface thereof and ranging along an axial direction; and a rotor core that includes a key that projects out on an inner circumferential side thereof, and is fitted in the keyway, wherein: a recessed portion that widens outward along a radial direction is formed at each of two sides of the key facing opposite each other along a circumferential direction at the rotor core; a planar portion is formed at a bottom area of the recessed portion so as to lessen stress occurring on each of the two sides of the key facing opposite each other along the circumferential direction; the planar portion is a straight contoured portion extending over a predetermined length along a direction perpendicular to a direction in which the key projects out, and a connecting side surface extending to an inner circumference of the rotor core is formed continuously to an end of the straight contoured portion located on a side opposite from a key side where a key is present; a first corner portion, connecting a side surface of the key with a key-side end of the straight contoured portion, is formed as a circular arc contoured portion; a second corner portion, connecting the end of the straight contoured portion, located on the side opposite from the key side, with the connecting side surface, is formed as a circular arc contoured portion; characterized in that dimensions and shapes of various portions are determined so as to substantially equalize stress levels in the first corner portion as a stress concentration portion and in the second corner portion as a stress concentration portion, wherein the second corner portion is located outward in a radial direction compared to the first corner portion. a rotating shaft with at least one keyway formed at an outer circumferential surface thereof and ranging along an axial direction; and a rotor core that includes a key that projects out on an inner circumferential side thereof, and is fitted in the keyway, wherein: a recessed portion that widens outward along a radial direction is formed at each of two sides of the key facing opposite each other along a circumferential direction at the rotor core; a planar portion is formed at a bottom area of the recessed portion so as to lessen stress occurring on each of the two sides of the key facing opposite each other along the circumferential direction; the planar portion is a straight contoured portion extending over a predetermined length along a direction perpendicular to a direction in which the key projects out, and a connecting side surface extending to an inner circumference of the rotor core is formed continuously to an end of the straight contoured portion located on a side opposite from a key side where a key is present; a first corner portion, connecting a side surface of the key with a key-side end of the straight contoured portion, is formed as a circular arc contoured portion; a second corner portion, connecting the end of the straight contoured portion, located on the side opposite from the key side, with the connecting side surface, is formed as a circular arc contoured portion; characterized in that dimensions and shapes of various portions are determined so as to substantially equalize stress levels in the first corner portion as a stress concentration portion and in the second corner portion as a stress concentration portion, wherein the second corner portion is located outward in a radial direction compared to the first corner portion. a recessed portion that widens outward along a radial direction is formed at each of two sides of the key facing opposite each other along a circumferential direction at the rotor core; a planar portion is formed at a bottom area of the recessed portion so as to lessen stress occurring on each of the two sides of the key facing opposite each other along the circumferential direction; the planar portion is a straight contoured portion extending over a predetermined length along a direction perpendicular to a direction in which the key projects out, and a connecting side surface extending to an inner circumference of the rotor core is formed continuously to an end of the straight contoured portion located on a side opposite from a key side where a key is present; a first corner portion, connecting a side surface of the key with a key-side end of the straight contoured portion, is formed as a circular arc contoured portion; a second corner portion, connecting the end of the straight contoured portion, located on the side opposite from the key side, with the connecting side surface, is formed as a circular arc contoured portion; characterized in that dimensions and shapes of various portions are determined so as to substantially equalize stress levels in the first corner portion as a stress concentration portion and in the second corner portion as a stress concentration portion, wherein the second corner portion is located outward in a radial direction compared to the first corner portion.