The invention discloses an optical scanning unit and a color image forming apparatus using the same. The color image forming apparatus obtaining a high quality color image with little write start misregistration among optical scanning devices, includes a plurality of optical scanning devices respectively including an incident optical unit guiding a light beam from a light source unit to a deflection unit, an imaging optical system allowing the light beam deflectively scanned by the deflection unit to be focused on a scanned surface, and an optical system for synchronous detection for adjusting image start timing on the scanned surface by using a part of the light beam deflectively scanned by the deflection unit. A ratio of a focal length of the optical system for synchronous detection to a focal length of the imaging optical system is set appropriately in a main scanning section of the plurality of optical scanning devices for obtaining a black color image and color images other than the black color image.