A light source unit includes a light source, a light source support to hold the light source, a fixing member which is attached to the light source support and includes a through-hole through which beams of light emitted from the light source pass, a lens holder inserted into the through-hole and attached to the fixing member by an adhesive; and a collimating lens to collimate the beams of light from the light source, wherein attachment surfaces of the adhesive for the lens holder has a tilted attachment surface tilted relative to the optical axis of the collimating lens.