An image forming apparatus is provided with a laser scanning unit having at least two laser diodes to form an electrostatic latent image on a photosensitive body with laser beams emitted from the laser diodes; an engine driving unit having at least one driving motor to drive a plurality of built-in units; and a control unit arranged to control one of the laser scanning unit and the engine driving unit so as to adjust a process speed of the engine driving unit or a scan speed of the laser scanning unit with a certain rate depending on a selected resolution in order to enhance a resolution of an image.