A vehicle interior illumination lamp 11 is formed by a function portion 12 and a design portion 13. Retaining claws 15 are formed on side surfaces of a base housing 41 of the design portion 13. Operating plates 33 are formed at a housing 21 of the function portion 12, and each operating plate 33 has retaining portions 34 with which retaining claws 51 on the design portion 13 (which can be fitted into the function portion 12) can be retainingly engaged, respectively. The operating plates 33 are pressed from the inside of a bulb receiving chamber 44 of the design portion 13 to be elastically deformed, thereby releasing the retaining engagement of the retaining portions 34 with the retaining claws 51.