An image forming apparatus has a first light controller to shift starts of image writing operations of light sources by a first time interval. A test data storage stores, beforehand, test image data having dot columns formed by one-dot images arranged in a row in a sub-scanning direction and at predetermined intervals in a main scanning direction. An operation controller controls an image carrier at a speed so that adjacent dot images in the sub-scanning direction overlap. A second light controller forms electrostatic latent test images on the image carrier based on timing signals with the image carrier moved by the operation controller, and draws electrostatic latent test images with different time intervals. A density measurer measures densities of the developed test images. A time interval adjusting section adjusts the time interval based on the time interval used to draw the electrostatic latent test image having the lowest measured density.