To realize a uniform pixel interval by making the shift of pixel interval variable by ±50% of a reference pixel frequency for each pixel when an f-θ lens is not used.

The reference frequency is divided, a phase for dividing the period of a frequency divided output equally is selected and outputted, a modulation pixel period for selecting the phase and outputting parallel pixel data depending on its output edge is set at the quantity of phase for the minimum pixel interval, and then the parallel data is converted into serial data depending on the minimum quantity of phase from the output edge.