A lamp holder 11 is made of white synthetic resin by molding such as injection molding. A reflection sheet 12 made of high-reflectance material such as silver or aluminum has a central portion 12a cut out and is affixed to an inner surface 11a of the lamp holder 11. The light reached the inner surface of the lamp holder 11 of light emitted from a lamp 1 is regularly reflected by the reflection sheet 12 and is diffusively reflected by the cutout portion of the reflection sheet 12. A considerable part of the light reflected on the white surface reaches the vicinity of electrode portions 1a of a discharge tube 1 to enter from an incidence end surface of a light guide in the vicinity of electrode portions 1a. Thus, the difference of light-supplying power between the central portion and the end portions (in the vicinity of the electrode portions 1a) of the lamp is compensated. When the reflection sheet made of a resilient material is used, the reflection sheet can be fixed utilizing the resiliency by putting the reflection sheet along the inner curved surface of the lamp holder. Instead of the reflection sheet, metal (silver or aluminum) may be evaporated on the inner surface of the lamp holder.