An image forming apparatus includes a latent image forming unit in which a light beam scans in a second scan region wider than a first scan region. The light beam is modulated in accordance with an image signal within the range of the first scan region for every scanning cycle and guided onto the effective image region to form a line latent image. A write timing adjuster adjusts a write start position for writing a latent image along the main scanning direction, based on a time difference between a first detection signal output when a first light beam scanning away from the effective image region moves passed an optical sensor, and a second detection signal output when a second light beam scanning toward the effective image region moves passed the optical sensor.