Boundary problem of diffraction electromagnetic waves on two-dimensional periodic metal gratings have been solved with taking into account finite permittivity of metal in optic range. Suggested technique is based on approach of approximate boundary conditions for thin dielectric layers. It is pointed out that dependence of magnitude of scattered field on wavelength has resonant behavior and resonant wavelength is many times higher than size of cell of grating. It is shown the possibility of designing of grating with high reflection coefficient at frequencies of plasmon resonance.