To provide a lamp unit in which rattling of a light-emitting element is prevented, a lamp unit includes: a holder member attached to an attachment object, a base board provided with a light-emitting element coupled on a main surface thereof; a retaining member that engages the holder member to sandwich the base board between the retaining member and the holder member such that the base board is retained at a prescribed position on the holder member; and a cover member that is at least partially light-transmissive and is attached to the holder member such that the cover member cooperates with the holder member to define a housing chamber for housing the base board and the retaining member.