A rotor yoke includes a substantially flat or annular flat cover portion coaxial or substantially coaxial with a rotational axis and a substantially cylindrical side wall portion extending from an outer circumference of the cover portion in a parallel or substantially parallel relationship with the rotational axis. The rotor yoke further includes a substantially annular flange portion extending radially outwards from an open end of the side wall portion. The cover portion and the side wall portion define a cup portion whose center of gravity deviates from the rotational axis in a plane perpendicular or substantially perpendicular to the rotational axis, and the flange portion has an outer circumferential circle whose center deviates from the rotational axis in a direction opposite to the deviation direction of the center of gravity of the cup portion.