When a semiconductor laser is energized, a virtual junction temperature rises to reduce a threshold current, which fluctuates a light output by the unit of a several microseconds, thereby causing printing irregularity. An object of the invention is to prevent the fluctuation in light output occurring due to the temperature change caused by the energization of the semiconductor laser with a simple circuit structure and under the driving condition of a small bias current. A constant voltage driving is performed with a circuit or device having negative resistance characteristics being arranged in series with the semiconductor laser. Thus, the reduction in light output of the semiconductor laser due to the temperature rise is compensated for by an increase in driving current thanks to a reduction in rising voltage of current/voltage characteristics.