Radiation generated by a charge moving along one of the prism borders is analyzed. The size of the object is assumed to be much larger than the wavelengths under consideration. We calculate the wave field outside the target using the "aperture method" which is applicable for any distances from the prism. We obtain the particular form of Stratton-Chu formulas for the case of the prism and develop an algorithm for the calculation of the radiation field outside the prism. Typical numerical results are demonstrated and discussed.