An image forming apparatus comprises a light generator (e.g., laser) generating a scanning light beam and a driver driving the light generator depending on a drive signal to be provided. In this apparatus, data formed of image information at each pixel of each line along a main scan direction of an image to be targeted are distributed into a plurality of strings (i.e., channels) of data (e.g., two strings). Concerning each pixel provided by data in each string, information indicative of both a start and an end of modulation (e.g., PWM) is then produced. By using the start and end information, a modulator performs modulation depending on the data of each pixel in each string, providing a modulated signal. The modulated signals from the plural strings are mutually synthesized at synchronizes timings, being provided as the drive signal and given to the driver.