An image-forming apparatus is disclosed that includes at least one imaging optical system forming an electrostatic latent image on the scanning surface of a medium moving in a sub scanning direction by scanning the scanning surface in a main scanning direction by periodically deflecting a laser beam with a rotary deflector, a pixel clock generation part generating a pixel clock signal for controlling the emission timing of the laser beam and changing the phase of the pixel clock signal, a region setting part setting the length of each of multiple regions based on image writing magnifications in a line in the main scanning direction, the regions being formed by dividing part of the scanning surface corresponding to the line in the main scanning direction, and a phase control part controlling the changing of the phase of the pixel clock signal region by region based on the image writing magnifications.