PURPOSE: To cut a steel sheet stuck with primary coat through one pass by determining the pulse frequency and pulse duty of a laser cutting machine while taking a thickness of steel sheet as parameter.

CONSTITUTION: While a steel sheet thickness information (sheet thickness) and an information of atmosphere temp. or steel sheet temp. are inputted in a control part 7, the control part 7 determines a pulse frequency Pf by a map 1 incorporated beforehand and a pulse duty Pi by a map 2 and operates a laser oscillator 2. Pulse frequency Pf=9×T+(680-780), pulse duty=0.75×T+(35-45), here, a max film thickness of primary coat is 18μm, an unit of Pf is Hz, T is a thickness of steel sheet in mm, the pulse duty is that the value defined by pulse width (time)/pulse interval (time) is expressed in terms of %.