1. An optical scanning unit comprising a plurality of optical scanning apparatuses each of which includes a light source unit, an incident optical system for allowing a light beam emerging from the light source unit to be made incident into a deflection unit, an imaging optical system for allowing the light beam deflected for scanning by a deflection surface of the deflection unit to be focused on a surface to be scanned, a synchronization detection element for detecting a part of the light beam deflected for scanning by the deflection surface of the deflection unit, and an optical system for synchronous detection for guiding the part of the light beam deflected for scanning by the deflection surface of the deflection unit to the synchronization detection element, wherein a focal length in a main scanning section of an optical system for synchronous detection of an optical scanning apparatus for forming a black color image among the plurality of optical scanning apparatus is denoted by fgBK (mm), and a focal length in the main scanning section of optical system for synchronous detection of optical scanning apparatus for forming color image other than the black color image is denoted by fgs (mm), and then the following condition is satisfied 
 fgs<fgBK.