PURPOSE: To make output timing of PWM value optimum by computing the transferring time of the tip of an image at exposure point to the development position according to the reduction or enlarge to determine the output starting timing of the development bias value.

CONSTITUTION: At first the distance from an exposure point to a development position and then the exposure width at a certain enlargement ratio is computed. Using the results of the computation, the time for transfer of the tip end of an image at the exposure point to the development position is computed based on the reduction or enlargement ratio. The alteration of the transfer time due to the fluctuation of the reduction or enlargement ratio computed by these processes is reflected upon the memory space quantity for delay generation of an FIFO structure which sends out PWM values every specified passage of time. Consequently, the PWM values saved in the memory space more prior are successively set out a the PWM values saved in the prescribed memory space are shifted to memory space with younger address number with the lapse of time. Consequently, even if the transfer time determined as a fixed time is fluctuated by the fluctuation of reduction or enlargement ratio, the transfer time is compensated properly.