PURPOSE: To improve a focus controlling accuracy by setting a focus setting ground point at a center of an amplitude of a displacement varied amount in a focus control of an optical disk original plate exposure apparatus.

CONSTITUTION: A high frequency component of an actuator current from a driver 11 of an actuator 5 is removed through a low-pass filter 23. A peak value, a bottom value are detected by a maximum value detector 24 and a minimum value detector 25, and input to a system controller 26. The peak and bottom values are multiplied by a deviation sensitivity of the actuator to obtain a displacement varied amount in an exposure range X of a glass original plate 7. Then, the actuator 5 is returned to an exposure starting position, a pulse number corresponding to the deviation amount is input to a pulse motor 21, and a vertically moving unit 20 is moved to correct a position.