A spindle motor of small size, thin type, high reliability, and high performance suited to an information recording and reproducing apparatus is presented. Specifically, a rotary disk is composed by integrally forming a disk board, and a rotary shaft of spindle motor, and a bearing is composed by forming dynamic pressure generating grooves in the rotary shaft of the rotary disk. As a result, small size and thin structure are realized, surface deflection of the disk during rotation of the spindle motor, and its axial center deflection are substantially decreased, and moreover the recording density is enhanced, the number of constituent parts is curtailed, and the cost is saved.