An image forming apparatus includes a light source, a light beam controlling mechanism, a sensor, and a signal controller. The light source emits parallel light beams with an angle smaller than 90null relative to a sub-scanning direction. The light beam controlling mechanism controls a scanning of the light beams. The sensor detects one of the light beams and generates a line synchronous signal. The signal controller delays the data streams and generates PLL clock signals divided into different clock signals having a same frequency and having phases sequentially varied. The signal controller selects one of the different clock signals and drives the light source with the selected clock signal.