A rotor hub for use in a motor installed into a recording disk driving device to spin a recording disk includes a convex portion, a disk placing portion around the convex portion, and the relief portion which is a substantially circular concave portion disposed between the convex portion and the disk placing portion. The recording disk, preferably a 1.8-inch or smaller type disk, having a center circular through-hole, is mounted on the rotor hub. When the recording disk is mounted on the rotor hub, with the bottom surface thereof abutting against a portion of the disk placing portion and an inner side surface of the center circular through-hole of the recording disk, an upper edge of the relief portion is arranged axially lower than an upper edge of the inner side surface of the center circular through-hole, and a bottom edge of the relief portion is arranged inward from an inner edge of the bottom surface of the recording disk.