An optical writer includes a scanning part causing a light beam from a light source to perform scanning on a photosensitive body; an image information obtaining part obtaining image information of an image; a dithering information obtaining part obtaining dithering information of the image; a pixel clock generating part generating a pixel clock signal; and a light emission control part to control the light source based on the image information in accordance with the pixel clock signal. The pixel clock generating part modulates the clock frequency of the pixel clock signal in accordance with phase shift information for correcting a change in the speed of the scanning on main scanning lines on the photosensitive body, and determines the timing of modulating the clock frequency based on the dithering information on a main scanning line basis so that the timing of modulating the clock frequency differs between the main scanning lines.