<p>A light beam position detecting device (38) detects the passage positions of a plurality of light beams for scanning the surface of a photosensitive drum. The light beam position detecting device (38) has a sensor pattern (S0) for generating an output which is continuously changed in a wide range with a variation in the passage position of the light beam in a sub-scanning direction perpendicular to a main scanning direction of the light beam. The sensor pattern (S0) precisely detects the relative scanning position of the light beam in a wide range. The passage positions of the plurality of light beams for scanning the surface of the photosensitive drum are controlled to a preset position based on the output of the sensor pattern (S0). <IMAGE></p>
