PURPOSE: To process an image even when the image is formed by using plural lasers without causing deviation of images by detecting an optical path of a laser beam and correcting image data depending on the optical path.

CONSTITUTION: An output signal from an image processing section 102 is given to a laser driver 201. The driver 201 controls a stimulated beam from lasers 202, 203 to emit the laser beam. Data for a black/white image are emitted from the laser unit 203 and the image is formed on a photoreceptor drum 214 via a decahedron mirror 205, an image forming lens 207 and a mirror 8. Furthermore, data for a red image are formed on the drum 214 from the laser unit 202 via a decahedron mirror 204, an image forming lens 206, and a mirror 209. In this case, the laser beam is given also to light receiving sections 210-213 and its output signal is given to the image processing section 102. The processing section 102 calculates the emission time from an input signal and calculates the optical path based thereon and corrects the image data depending on the optical path.