A multi-beam scanning device is disclosed that is able to realize a stable and small beam spot and realize stable scanning line intervals between plural light beams. The multi-beam optical scanning device includes a first optical system which has a first lens for coupling the light beams from the light sources, and a second lens which is an anamorphic element having power at least in a sub scanning direction and for guiding the light beams from the first lens to the deflection unit; and a second optical system. At least the second lens includes a diffracting surface having power. Magnifications of the whole optical system in a main scanning direction (βm0) and in a sub scanning direction (βs0), and a magnification of the second optical system (βs2) in a sub scanning direction satisfy |βm0|>|βs0|, and |βs0|>|βs2|, and the power of the diffracting surface of the first lens in the main scanning direction (P1m), that in the sub scanning direction (P1s), and the power of the diffracting surface of the second lens in the sub scanning direction (P2s) satisfy |P2s|>|P1m| and |P2s|>|P1s|.