The electromagnetic radiation scattering by diffraction grating possesses many peculiarities under condition of resonance diffraction  *, where  * is the grating wavevector,  * is tangential wavevector's component of the incident wave, m is integer. E.g., these are great increase of the resonance spectra amplitude, transformation of polarization, suppression of specular reflection. These phenomena have the same nature as well-known Wood anomalies, and they are caused by resonance of the grazing spectra and eigenmode of a highly conductive surface — the surface electromagnetic wave (SEW). Many works in recent years have been devoted to experimental and theoretical investigations of resonance diffraction (see, e.g., [1-3]). In order to explain strong resonance effects complicated numerical calculations are used [4, 5]. The essential simplification is made in this paper. It is based on modified perturbation theory (see [6, 7]).