To control an optical output to be constant without deteriorating an extinction ratio of the optical output, when the differential quantum efficiency and the threshold current of an LD deteriorate.

An LD drive section 2 supplies a drive current, in response to input data to an LD 3. A monitor section 4 monitors an optical output of the LD, and an APC circuit 5 controls the drive current in response to a monitored average voltage 51. A consecutive 0 detector 10 activates a level comparator control switch 12, when receiving consecutive 0 data in excess of a prescribed length to operate a level comparator 11, where an optical monitor voltage 41 is compared with a reference voltage. When an optical output with respect to data '0' exceeds a reference value, an extinction ratio deterioration signal 111 is generated and a data control section 13 increases the drive current 21 to increase the optical output, and then the voltage 51 is increased. The APC circuit 6 decreases the LD bias current, by using the voltage 51 for a reference to obtain the optical output with less extinction ratio deterioration and waveform distortion.