An exposing unit has a plurality of light emitters arranged in a main scanning direction. The exposing unit is configured to form a scanning line by exposing an image surface in the main scanning direction, and to form a plurality of pixels arrayed in the main scanning direction by sequentially forming a plurality of scanning lines shifted in a sub-scanning direction. A clock generator is configured to generate frequency-spread clocks having frequency that is spread with a particular modulation cycle. A driver is configured to drive the exposing unit to emit light during a lighting period based on the frequency-spread clocks. The driver sets an interval of exposure start time of the plurality of scanning lines. At least one interval is different from a reference period obtained by dividing a period for forming the pixels arrayed in the main scanning direction by a number of the plurality of scanning lines.