A concept of indoor solar illumination is described and designed. The solar illumination system is composed of a none tracking primary reflector and controlling mirrors. The primary reflector is in form of a truncated tetrahedral pyramid covered by a thin flat glass sheet on the wide opening of the pyramid to prevent dust to accumulate on the reflecting sides of the pyramid besides preventing dust from coming inside the room. The controlling mirrors are plane and rectangular. Each part of the solar illumination system is optically suited and compatible with other parts to realize high efficiency. The optical design is conducted for Baghdad city at interior building in two solstice days over the year. Research results showed that the design of the solar system is achieved on the base of minimum and maximum solar illuminance level in 21 June. Tetrahedral pyramid's dimensions in a question as a solar concentrator for this paper are: concentration ratio is 3 for 90 cm entrance opening, 25° for half angle and 5° for controlling mirrors.