A non iterative method for the identification of heat sources in a solid body is proposed by combining analytical and numerical approaches. In this method, imaginary heat sources located on the temperature observation points are introduced. Then the relationship between the observed temperature and the response temperature of the imaginary heat source is derived from the symmetry of Green's function, which gives a formal solution of the heat conduction problem. Using this relationship, for example, it is shown that the location and the intensity of a single heat source can be identified from only three observed temperatures, by executing four numerical simulations, in the case of a two-dimensional problem. In addition, the relationship is easily expanded to three-dimensional and to multi-heat-source problems. Some computational experiments demonstrate the capability of the proposed method.