PURPOSE: To shorten the time for setting the scanning position of a laser light beam at the center of an output means which outputs a reference position signal with exposure by changing an exposing position on the output means to a direction orthogonal to a scanning direction and deciding the central position in accordance with an output signal corresponding to a changing position.

CONSTITUTION: An image carrier is exposed to the laser light beam generated in accordance with the data from an external device by an exposing means 12 to form an image, and the output means 63 which is provided near the image carrier and has such structure that width near the central position with respect to the direction orthogonal to the scanning direction of the laser light beam is different from other parts outputs the reference position signal with the exposure by the exposing means 12. Then, the laser light beam exposing position on the output means 63 by the exposing means 12 is changed to the direction orthogonal to the scanning direction by a changing means 65, and it is decided that the exposing position by the means 12 is the central position in accordance with the output signal from the output means 63 corresponding to the changing position. Thus, the setting time in the case of setting the scanning position of the laser light beam at the center of the output means 63 is shortened.