In a laser beam printer, even when an inexpensive CPU having low throughput capacity is used, stable power can be obtained by following a sudden change in temperature of a laser chip. A control unit sets a scanning cycle for performing the automatic power control based on ambient temperature of a photoconductive drum and the number of continuously-printed sheets. Then, the control unit output a horizontal synchronizing signal to a laser driving circuit and causes a laser scan unit to output a scanning laser beam. Substantially, when scanning corresponds the scanning cycle, the automatic power control is performed based on an output signal of a photodiode. When scanning does not correspond to the scanning cycle, a horizontal synchronizing signal of the next line is output. Whereby, the scanning cycle for performing the automatic power control according to a change in temperature of the laser chip, thereby reducing loads applied to the control unit.