In order to improve the adhesion of a magnetic steel sheet with no film of an inorganic material on the surface, in particular to a tension-imparted insulating film, an anodic treatment is executed in an aqueous solution of a silicate prior to forming the insulating film. Thus, a thin silicic film having a good adhesion to the insulating film is firmly formed on the surface of the surface of the steel sheet. This makes it possible to intimately and favorably form a tension-imparting insulating film on a grain oriented magnetic steel sheet to lower the iron loss of the grain oriented magnetic steel sheet. Even for an insulating film on a magnetic steel sheet which is not of the tension-imparting type, it is possible to improve the heat resistance of the film and to improve the insulating properties since the thickness of the film can be increased.