Optical scanning device and image forming apparatus

An optical scanning device comprising a multi-beam light source having a plurality of light emitting elements aligned in parallel in a predetermined linear direction, a rotary polygon mirror, and a reflection mirror along a rotary axis direction of the rotary polygon mirror, a plurality of light beams emitted from each of the plurality of light emitting elements being reflected by the reflection mirror, the plurality of light beams reflected by the reflection mirror being made incident to the rotary polygon mirror, and the plurality of light beams reflected by the rotary polygon mirror being emitted toward an object to be scanned, wherein the multi-beam light source is capable of rotational adjustment around a rotational axis along an emission direction of the plurality of light beams, and a rotational direction of the multi-beam light source around the rotational axis is made different to a side where a sub-scanning width in a sub-scanning direction of the plurality of light beams is made smaller between a case of one-side incidence in which the plurality of light beams are made incident to the rotary polygon mirror from either one of sides in the rotary axis direction of the rotary polygon mirror and a case of the other-side incidence in which the plurality of light beams are made incident to the rotary polygon mirror from the other side.