PURPOSE: To restrain deformation at cutting off by providing intermittent slit cut parts leaving uncut parts along prescribed cutting lines of a steel plate, and after drilling on prescribed positions by a NC drilling machine, cutting the uncut parts on the cutting lines so as to manufacture a plurality of perforated steel plates.

CONSTITUTION: Intermittently cut slits 2 are provided along cutting lines of prescribed width, flange width B, web width B, on a large-sized steel plate 1 of length L and width B. Namely uncut parts are provided on both end parts, the vicinity of bolt hole positions, and the positions where restraint is exhibited so as not to generate deformation inside and outside of the plane of steel plate 1 at cutting the slits 2. The position of slit 2, the length S1 of slit, and the length S2 of uncut part are decided by a handling method in case of cutting the uncut parts of the steel plate 1 after cutting the slits or moving them for assembling into a shaped steel. Next after drilling bolt holes on the end parts of flange and web materials by a NC drilling machine, the uncut parts, the surplus length parts are cut off by a gas or plasma cutter so as to manufacture a long steel plate.