PURPOSE:To facilitate the severing operation of a steel belt and to attain favorable one-turn cut type wound core by a method wherein the steel belt is cut such that its widthwise both side parts are completely cut off and a stepped uncompletely cut-off part is formed in the widthwise central part as a notch. CONSTITUTION:A steel belt 1 is stopped for each predtermined length and an upper blade 6 of a cutter is descended, thereby to completely cut off both side parts 14 thereof with both side blades 10, 10. At the same time, a center blade 9 is descended to press the center part of the steel belt into the stepped semi-cut state, and its descent is stopped by striking against an upper blade stopper 8 through the steel belt 1. An uncompletely cut-off part 13 in the step-like shape is formed in the center part of the steel belt 1. When a core made of the steel belt 1 wound therearound is pressed and formed into the square form, the steel belt 1 is broken at each notch 15 and severed for each one-turn. Thus, the force necessary for breakage is small and breakage of the steel belt becomes easy, thereby breaking the steel belt 1 accurately without forcing.