A simulator of heat radiation energy which includes acquiring 3D spatial data including a plurality of solid objects where the 3D space is defined by a plurality of grids; calculating the form coefficient in all combinations of arbitrary two grids out of the plurality of grids; acquiring the attribute data of each grid including the area, reflectivity, and temperature of the grid; calculating the sum of heat radiation energy by radiation of a grid itself and heat radiation energy by reflection of heat radiation from each grid as heat radiation energy of every grid on the basis of the area of the grid, the form coefficient, the reflectivity of the grid and the temperature of the grid; and outputting the calculation results of heat radiation energy of every grid to an output unit.