An image forming device includes a photosensitive body, an electrostatic charging device which electrostatically charges the photosensitive body, a light exposure device which image-light-exposes a moving surface of the photosensitive body so that an electrostatic latent image is formed on the photosensitive body, and a developing device which makes visible the electrostatic latent image as a toner image. The light exposure device forms a beam spot on the moving surface of the photosensitive body by selectively irradiating a light beam corresponding to image data in a state where parts of two beam spots neighboring each other in a sub scanning direction overlap each other in the sub scanning direction. A summation of light energies when the two beam spots are formed on the surface of the photosensitive body with a time interval is set to be smaller than a summation of light energies when two beam spots are simultaneously formed on the surface of the photosensitive body.