To accurately detect the step-out even when the step-out has occurred at any position of a rotor by detecting the information of the rotary board, wherein the rotation is performed in relation to the rotor, the board is divided equally in the circumference direction and a plurality of signal patterns, in which binary signals are recorded at the respective duty ratio, are sequentially arranged.

A male-screw shaft 4 is rotated by the drive of a stepping motor 3, and a lens holder 1 is moved in the parallel direction with the optical axis in correspondence. The moving amount is determined by the number of the drive pulses of the stepping motor 3. The problem occurs, however, when the load is increased by some reasons and the stepping motor 3 is driven under the state, wherein the step-out has occurred. Therefore, the presence or absence of the step-out is detected by a slit plate 5 of the rotary board and a photointerrupter 6 of a detecting means. Therefore, the step-out of the rotor can be securely detected with the simple constitution.