An exposing unit forms a scanning line by exposing an image surface in a main scanning direction, and forms 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 generates frequency-spread clocks having frequency that is spread with a particular modulation period and that changes in accordance with a modulated waveform corresponding to the particular modulation period. A driver drives the exposing unit to emit light during light emitting time that is determined from the frequency-spread clocks. The plurality of scanning lines is formed with respective phases of the modulated waveform. The phases corresponding to the plurality of scanning lines are shifted from each other for compensating, among the plurality of scanning lines, deviations of the light emitting time relative to a reference value, for each of the plurality of pixels.