There is provided a light source unit comprising a sealing member, a holding member on which an abutment surface is formed, which has an engaging portion and which holds a part, a case on which an abutted surface is formed which is brought into abutment with the abutment surface via the sealing member, which has an engaged portion which is brought into engagement with the engaging portion and which holds the holding member, wherein the abutment surface of the holding member and the abutted surface of the case face each other and are inclined relative to a holding surface which holds the part, and the abutted surface of the case is inclined so as to face a side where the holding member is inserted.