To realize a print with a low density by increasing gradation number of the density.

Comparison is conducted only for a period, when each ramp wave has superior linearity by comparing a level of an analog signal with a signal level for the comparison period other than a change start period, immediately after start of a signal level change in the ramp waves R1-R3. No comparison is made for a period, when a voltage just after start of the ramp wave is fluctuated and a pulse with superior linearity can be outputted even at a low code. Controlling laser oscillator outputting a laser beam to be emitted to a photoconductive drum of a printer with each output pulse can increase density gradation number to control the print density, in response to an output of the laser oscillator so as to realize a print with a low density.