Multi-beam optical scanning device

It is an object of the present invention to realize a multi-beam optical scanning device which is suitable for high speed and high recording density by almost completely offsetting and correcting a deviation of focusing positions in a main scanning direction of plural spots in an entire surface to be scanned without deteriorating focusing properties at all. The multi-beam optical scanning device includes: a light source device having plural luminescence parts; a rotating polygon mirror ; a first optical system which converts plural light beams into convergent light beams or divergent light beams; and a third optical system which guides the light beams onto a surface to be scanned of a drum shape, and when it is assumed that a maximum value of a positional deviation amount generated in a first direction is δY, a maximum value of a positional deviation amount generated in a second direction is δY, and a maximum value of a positional deviation amount generated in a third direction is δY, conditional expression (1) is satisfied.