a slower rate than at the surface of the drum longitudinal ends 1002 which is more distant from the polygon mirror center 1001 the f u03b8 lens 1005 however corrects this tendency and plays a role of causing the laser beam to pass at a constant speed on the drum surface under the effect of refraction of the light the laser detector 1004 arranged by the side of the photosensitive drum 1002 functions as a sensor for detecting a reference position of the laser and determines an irradiating period of image information from the laser device 1003 each face of the polygon mirror 1001 corresponds to one scanning line in which each face uses signals from the laser detector 1004 for start timing signals for the individual lines in this embodiment as shown in fig1 the odd numbered lines are divided into eight segments such as segments a to h to control the laser the even numbered lines are divided into seven segments such as segments a to g to control the laser more specifically fifteen average values of the amount of correction for the individual segments are stored in a register to make a correction within segments with a constant frequency referring to fig1 frequencies within the segments are stored in the register 1403 via the cpu bus the counter 1404 cleared by a start reference signal generated by the laser detector.