PURPOSE: To evade a failure in focus drawing at the time of focus control by controlling a position of a lens according to an output of an adder.

CONSTITUTION: By a focus controller of an optical disk device, the position of the lens 4 converting a light beam 3 is controlled so as to focus the light beam 3 on an optical disk 2. This device is provided with a voltage generation means 70 generating an offset signal for moving the lens 4 to the position where the light beam 3 is focussed on the optical disk 2 only when the light quantity of a reflected light or a transmitted light from the optical disk 2 is smaller than a prescribed value when the optical disk 2 is irradiated with the light beam 3, and the adder 60 adding the offset signal to a focus error signal. Then, the device is constituted so that the position of the lens 4 is controlled according to the output on the adder 60. In such a manner, since the lens 4 is controlled to be at the position where the light beam 3 is focussed on the optical disk 2, the failure in the focus drawing is evaded at the time of focus control.