Supported by the boundary conditions of the scattering field related to the incident field through the Fresnel coefficients, and by the reciprocity theorem and the equivalent field principle, the multiple Kirchhoff integral method for metal blazed grating is developed. Under the conditions of Littrow mounting and fixed angular deviation mounting, illuminated by plane wave, the efficiency of ideal conductor blazed gratings is calculated. The stability and the self-consistency of this method are discussed.