An image forming apparatus includes: a latent image carrier that includes an effective image region which has a predetermined width along a main scanning direction; a latent image forming unit that has a structure in which a deflection mirror surface makes a light beam scan in a second scan region which is wider than a first scan region which corresponds to the effective image region, that modulates the light beam in accordance with an image signal within the range of the first scan region for every scanning cycle, and that guides the modulated light beam onto the effective image region, thereby forming a line latent image which corresponds to the image signal; an optical sensor that detects a scanning light beam, which moves outside the first scan region within the second scan region, to output a signal; and a write timing adjuster that controls the timing of start modulating the light beam and accordingly adjusts a write start position for start writing a latent image along the main scanning direction, based on a time difference between a first detection signal, which the optical sensor outputs as a first light beam scanning away from the effective image region moves passed the optical sensor, and a second detection signal, which the optical sensor outputs after outputting the first detection signal when a second light beam scanning toward the effective image region moves passed the optical sensor.