To form a slot insulator with high efficiency and improved impregnation.

In the slot insulator which is inserted for the purpose of insulation to ground between the coil 6 and the iron core 8 of an electric apparatus impregnated with insulating oil by drop impregnation, a tape 2 for slot insulation is wound a little longer than the length of a slot by half overlap winding on a bar which has the sectional form of such dimension that the thickness of the slot insulator, the thickness of a flaw preventive sheet 3, and the margin space 4 are subtracted from the dimension of the sectional form of the slot and besides is longer than the length of the slot. Then, the flaw preventive sheet 3 is wound around it, and after insertion from the top or side of the slot 1, the flaw preventive sheet 3 is pulled out for removal, and the slot insulating tape 2 at the section applicable to the center of the slot part is cut in the longitudinal direction of the slot, whereby the slot insulating tape 2 separates from the bar by the firmness of itself, and sticks fast to the inner face of the slot thereby forming the slot insulator.