In recent years, the electric power consumption is needed to reconsider due to global warming and other environmental issues. In order to reduce electric power consumption in office buildings and houses, new illumination system may be a good step to start as illumination system uses about 20-30% electric power in any offices or houses. Therefore, to start saving electricity will be an effective method to reduce power consumption. Natural light can be an assistive tool for interior illumination system. We have developed a simulation method which uses Monte Carlo method in order to imitate any office or house and its illumination system. We have proposed to use a window shade reflector in dazzling sunny bright day at summer to use the natural light. From our calculation, no reflector is necessary for cloudy day. By using the natural light which can satisfy the JIS illumination standard, we can save at least 20-30% of the total electric power consumption. Moreover, no extra manmade light is necessary in summer’s sunny bright day if we use a reflector in the window. However, further calculations are needed to consider the other season’s daylight for interior illumination system.