An image forming apparatus which includes image forming units of a plurality of colors, each having an image carrier and a light beam scanning portion for polarizing irradiated laser beam by a polygon mirror and forming an electrostatic latent image on the image carrier, and forms a color image by superposing images formed by the image forming unit, includes a first control portion which controls the velocity of rotation of one polygon mirror in one image forming unit, a second control portion which controls rotation of another polygon mirror by velocity control or phase control in another image forming unit, and a control signal generation portion which, when the rotation velocity of another polygon mirror reaches a target velocity by velocity control of the second control portion, generates a rotation reference signal based on the driving information of one polygon mirror driven by velocity control of the first control portion, or detection information of the laser beam polarized by one polygon mirror, wherein the second control portion controls, when the control signal generation portion outputs the rotation reference signal, the rotational phase of another polygon mirror in order to achieve phase locking with the rotation reference signal.