An object of the invention is to provide an estimating system of heat load in a perimeter zone which can detect heat load in a perimeter zone with high accuracy without providing an exclusive detecting device for detecting heat load, such as a radiation thermometer. The estimating system of heat load in a perimeter zone in the invention includes a light-transmitting solar cell which is provided on a window face; and heat load estimating means for estimating heat load in a perimeter zone based on output characteristics of the solar cell, in which the heat load estimating means obtains an intensity of solar radiation from a window face and a temperature of the solar cell, from a short circuit current value and an open voltage value due to power generation of the solar cell, and calculates a mean radiant temperature in the perimeter zone using the intensity of solar radiation and the temperature of the solar cell.
