Thermal radiation emitted from flat facet edges of an aluminum bar has been experimentally investigated. It is established that the radiation is predominantly p-polarized and has the pattern with a pronounced maximum oriented at an angle of a few degrees to the plane of the facet. In addition, the magnitude of this maximum increases with the facet length size and tends to saturation as the length increases. The facts suggest that this phenomenon occurs due to diffraction of infrared surface plasmon-polaritons (SPPs) generated by phonons of the sample. This assumption is supported by a cubic dependence of the radiation intensity on temperature, which is typical for thermally generated SPPs (TSPPs). The analytical model for computing the spectrum and the integral intensity of the entire set of TSPPs arriving to the edge of their sources line has been developed as well.