PURPOSE: To smoothly lock a focus actuator to a focus servo state by setting the initial value of a search signal for attaining a focused state fetched into a focus servo loop to the value of a focus error signal which exists at the moment when the search signal is fetched into the loop.

CONSTITUTION: When focus servo is applied to an optical disk device, a pull-in driving signal which moves a focus actuator to a focus attainable area is first supplied to a focus search circuit and, when the focus actuator enters the focus attainable area, a focus attain search signal Sf whose initializing level is set at the same level as that of a focus error signal ef at the moment is formed. Then the focus attain search signal is supplied to the focus actuator together with the focus error signal ef. Therefore, the focus actuator can be locked smoothly to a focus servo state even if the kinetic characteristics of the focus actuator extensively fluctuate due to the attitude of the optical disk device, static friction, etc.