When an APC selection signal FA is set to H level to start an APC operation, a monitor voltage VA is immediately generated from a monitor voltage generating circuit 16A, and an error voltage VEA corresponding to the difference between the monitor voltage VA and a reference voltage VP is output from an error amplifying circuit 18A. However, until the monitor voltage VA reaches the vicinity of the reference voltage VP, a S/H circuit 20A is held in a hold mode, and a laser diode LDA is driven at a switching current ISA with the same current value as before the APC operation start. After the monitor voltage VA reaches the vicinity of the reference voltage VP, the S/H circuit 20A is switched to a sampling mode, and a feedback control operation of a closed loop is started.