PURPOSE: To regulate a focus servo circuit in a short time by calculating a balance constant and an offset constant based on the signal from optical detection elements.

CONSTITUTION: An adjusting section 10 comprises an offset detecting section 10a, a signal detecting section 10b, a balance constant setting section 10c, and an offset constant setting section 10d and adjusts a balance constant Cb and an offset constant Co. The offset detecting section 10a has a function for detecting the output voltages from first and second optical detection elements 3a, 3b under a state where no light is incident on an optical detecting section 3 and the signal detecting section 10b has a function for detecting the output voltages from first and second optical detection elements 3a, 3b. The balance constant setting section 10c has a function for setting a balance constant Cb and the offset constant setting section 10d has a function for setting an offset signal Co. Since the balance constant Cb and the offset constant Co are determined based on the output signals from first and second optical detection elements 3a, 3b, they can be determined by a single adjusting operation and a focus servo circuit can be regulated in a short time.