A multi-beam light source unit includes a light source section having a plurality of semiconductor lasers, a plurality of collimating lenses for collimating light beams emitted from the semiconductor lasers, and a supporting member for supporting the plurality of semiconductor lasers and the plurality of collimating lenses arranged in a main scanning direction. There is a relation expressed by D/d.noteq.1 relating to a distance D between adjacent ones of the semiconductor lasers and a distance d of adjacent ones of the collimating lenses, and the light source section is supported so that it can be angularly adjusted so as to change a rotational position of the light source section including the semiconductor lasers and the collimating lenses.