PURPOSE:To form a film having good appearance and uniformity in the stage of forming a forsterite base insulating film on a cold-rolled grain oriented silicon steel sheet by controlling the amt. of the oxide film to be formed in decarburization annealing according to the residual amt. of the Na compd. originating in degreasing and washing. CONSTITUTION:A grain oriented silicon steel sheet which contains 2-4% Si and is cold-rolled to a final sheet thickness is subjected to alkali degreasing, washing and decarburization annealing in a wet hydrogen atmosphere to form an SiO2 film thereon. An annealing separating agent consisting essentially of MgO is coated on said sheet and the sheet is coiled. The coil is annealed at 1,100- 1,200 deg.C in a hydrogen atmosphere to form a forsterite base insulating film on the surface of the steel sheet. The forming amt. of the oxide film generated by the decarburization annealing according to the amt. of the Na compd. originating in the degreasing and washing and remaining on the surface of the steel plate is so controlled as to satisfy y=1.0-2.0g/m<2>, (0.43x)<2>+(0.50y)<2=1 when the amt. of the Na to be stuck for each 1m<2> unit area on both surfaces of the steel sheet is designated as x mg and the oxygen weight in the oxide film as y g.