To avoid the reaching of the thermal effect in laser beam trimming on a part other than a trimming groove by blowing the gaseous cooling medium to the vicinity of the part among a resistor film and a protecting film, to which laser beam is projected, and projecting the laser beam under this state.

When a trimming groove 2 is cut and provided in a spiral shape by a laser beam trimming method for a cylindrical resistor 1, the cooling medium such as air, which is compressed by a compressor 8 and cooled to the temperature lower than atmospheric temperature by a cooler 9, is blown to the vicinity of a part, to which the laser beam is projected, at the outer surface of the cylindrical resistor 1 through a nozzle 10. Thus, among a resistor film and a protecting film, the vicinity of a part, to which the laser beam is projected, can be sequentially cooled, and the range affected by the thermal effect in the laser beam trimming can be made small.