A synchronizing circuit and a timing adjusting circuit set a position of an exposure area by beam scanning as an additional value of a first setting value set in dot unit corresponding to the scanning of a pixel clock period and a second setting value set in tap unit corresponding to the scanning of a period smaller than the pixel clock period. A CPU controls the synchronizing circuit and the timing adjusting circuit so that the exposed position becomes an objective position. The CPU changes the second setting value to a first predetermined value by increasing the first setting value by one dot, in the case that the additional value is increased to reach a first threshold. Further, the CPU changes the second setting value to a second predetermined value by decreasing the first setting value by one dot, in the case that the additional value of the synchronizing circuit and the timing adjusting circuit in which the first setting value is increased by one dot is decreased to reach a second threshold smaller than the first threshold.