A multi-beam scanning device includes a plurality of light sources which emit multiple beams. A plurality of coupling lenses couple the multiple beams from the light sources. A converging lens device converts the coupled beams into converging beams, the converging beams being elongated in a direction corresponding to a main scanning direction and converging only in a direction corresponding to a sub-scanning direction, at least two of the converging beams forming a difference angle therebetween. A rotary deflector has a reflection surface rotated about a fixed rotation axis. The reflection surface deflects the converging beams while the reflection surface is rotated. A scanning lens device scans an image surface in the main scanning direction at a substantially uniform velocity by focusing each deflected beam into a beam spot on the image surface. The multi-beam scanning device is configured to satisfy desired conditions to allow the multi-beam scanning device to achieve excellent optical characteristics without using a polarization beam splitter. Further, an image forming system and method which uses such a multi-beam scanning device is provided.