The mechanism of the appearance of the diffraction nonreciprocity of counterpropagating waves in a ring gas laser is examined. Formulas were obtained for the difference between their frequencies and intensities, the dependences of which on the detuning agree with those obtained earlier and with experiment. Explicit dependences of the differences between the frequencies and intensities of the counterpropagating waves on the cross sections of the aperture and of the Gaussian beams incident on the latter were found.