A multibeam light source unit, a method of adjusting the same, a light scanning unit, and an image forming apparatus are provided. The multibeam light source unit includes light sources that emit multiple beams, a flange that includes installation surface on which holder installation holes are provided, and middle holders on which a light source is installed. The middle holder includes a light source mounting unit, a cylinder-shape insertion unit that is inserted into a holder installation hole, and fixing protrusion units protruding outside from a light source mounting unit and fixed to the flange, and a jig coupling unit that is coupled with a jig when adjustment is performed.