Lamp unit and vehicle door equipped with lamp unit

A lamp unit, comprising:

• a holder member configured to be attached to an attachment object;
• a base board provided with a light-emitting element 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,
• wherein the holder member has a housing recess for housing the base board and the retaining member,
• an inner surface of the holder member is provided with at least one engagement portion for locking the retaining member,
• the retaining member has a plate-like shape and is provided with an outer frame formed along an edge portion of the base board and a light passing hole that extends through the retaining member at a position corresponding to the light-emitting element inside the outer frame, and
• the retaining member is provided with first projections respectively projecting from at least two portions of an outer peripheral surface of the outer frame that face away from each other such that each first projection projects toward a wall surface defining the housing recess.