To promote energy saving for office lighting, effective utilization of daylight illumination is necessary. It is very important to evaluate the brightness of a room taht is illuminated by both daylight and ceiling lamps. In this case, even if the average horizontal illuminance is kept constant, it is difficult to estimate the brightness perception of the room because of the very high intensity of daylight and the uneven distribution of light flux. In this paper, we investigated the effect of window size and intensity of daylight on the brightness perception of an office room with a window on one side. We examined the brightness perception and expressed it as a function of average horizontal illuminance attributed to the daylight and to the ceiling lamps.