The self-consistent integral equation for the field distribution of the resonant modes in a concave (plane) grating-mirror stable resonator is solved in the limit of infinite Fresnel numbers and under Littrow (collimate) configuration. Under the conditions above mentioned the field distribution is approximately independent of the blazing angles of the grating. In the direction parrallel to the grooves of the grating (y direction) the inclined grating can be equivalent to the concave mirror which has the radius of curvature ry-r1·cosθ and is perpendicular to the axis of the resonator (z axis). In the x direction the field distribution can be described in terms of Hermit-Gaussian function and the perturbation term. The perturbation term which is proportional to tangent of the incline of the grating introduces astigmatism to the field. When the perturbation can be neglected, the inclined grating would be equivalent to a concave mirror of radius rx=r1-cos3θ, which is also perpendicular to the axes of the resonator.The resonant condition, the stabilty condition, the field distribution and other charecteristies of the field distribution are obtained.