The direct sunlight entering a room through a window produces high luminance areas on the floor or walls, which makes us uncomfortable. We conducted an experiment to make subjects experience a series of illumination using actual daylight in an experimental chamber. As the result, when the prime luminance ratio of an area to the surroundings is more than 1.2 to 1, subjects perceived it as a high luminance area. Subjective glare sensation caused by a high luminance area could be explained with the area luminance, the solid angle of the area and the prime luminance ratio of the area to the surroundings.