A light source unit includes a light source including a light emitting portion, a light source holding member holding the light source, a light transmission member including an incident portion wherein light from the light emitting portion is to enter the incident portion, a light transmission member holding member holding the light transmission member, a positioning portion provided on the light transmission member holding member and abutted on the light source to position the light transmission member holding member and the light source to position the incident portion and the light emitting portion, and an adjustment joining mechanism configured to join the light transmission member holding member and the light source holding member to each other so as to adjust the relative positions thereof, and joining them to each other in a state where the light source and the positioning portion abut on each other.