PURPOSE: To produce a grain oriented silicon steel sheet having an extremely small iron loss by finishing the surface of a finish-annealed steel sheet from which surface oxides are removed near the center line thereof to a specular surface then forming microstrains thereto to form a mixed phase in which fine crystal grain groups mixedly exist and further forming an insulating film consisting of a specific material.

CONSTITUTION: After a silicon steel contg. 2W4% Si is hot rolled, the steel is subjected to homogenization annealing, then to cold rolling to form the steel sheet having the final sheet thickness. The sheet is subjected to a decarburization primary recrystallization annealing treatment in succession of the above; thereafter, a separating agent for annealing essentially consisting of MgO is coated thereon and the sheet is subjected to secondary recrystallization annealing and further to purification annealing in dry hydrogen. The surface is finished to a specular surface having ≤0.4μm center line average height by polishing and thereafter the local microstrains are formed thereto. The ultra-thin tensile film which has ≤1.0mm fine crystal grain groups mixedly existing in the secondary recrystal groups and consists of the nitride, carbide, boride, etc. of various metals is successively formed at ≥500°C by a CVD method, etc. and finally the insulating film essentially consisting of a phosphate and colloidal silica is coated thereon and is baked, by which the silicon steel sheet having the extremely small iron loss is produced.