About 20-30% of the power consumption in an office or a workshop depends on the illumination system. Hence, the use of renewable energy such as sunlight is very important in illumination system on the basis of energy saving. But the design of nature helping illumination system is not easy to plan due to its difficulty of various conditions. We have developed a simulation system by using Monte Carlo method to calculate the interior illumination distribution which can also be aided with natural light system. Our simulation method have focused well the illuminance distribution of a room for both man made light and natural light.