The brightness of a room interior can be roughly determined from its luminance. However, the luminance of a room interior is not even, but rather is an amalgam of high-luminance areas and low-luminance areas. In this research we intended to determine the effect of luminance distribution on brightness. First, we conducted a experiment to estimate the brightness of stimuli that are of the different luminance distribution. Next, a experiment that examined the relationship between the characteristics of luminance variations and brightness in interior environment was conducted. As a result, the brightness in a room interior depended not just on luminance of visual field but also on the distribution of field luminance, and that although luminance distribution that originates in the varying reflectance of walls and floors does not affect brightness to a great extent, the luminance distribution resulting from non-uniform lighting caused an interior to seem darker.