An optical scanning device comprises a multi-faceted rotating polygon mirror for deflecting an incident beam in main scanning direction, a first light source for bringing a first beam into incidence on the polygon mirror at an angle of +.alpha. with respect to a central line connecting from a rotation center of the polygon mirror to a central position of a scanned surface, the first light source arranged such that the first beam is deflected at an angle of -2.alpha. by while the polygon mirror rotates at an angle of .alpha., a second light source for bringing a second beam into incidence on the polygon mirror at an angle of -.alpha. with respect to the central line, the second light source arranged such that the second beam is deflected at an angle of -2.alpha. by while the polygon mirror rotates at an angle of .alpha., a first reflecting mirror for scanning the scanned surface by reflecting the deflected first beam, the first reflecting mirror arranged so one end portion thereof as to cross over the central line and to avoid an unnecessary beam from reflection faces adjoining to a normal reflection face of the polygon mirror, and a second reflecting mirror for scanning the scanned surface by reflecting the deflected second beam.