PURPOSE: To allow both of a programer and an operator to easily grasp the machining conditions and to improve working efficiency, machining efficiency and machining quality by automatically setting a laser beam output condition and the assisting gas pressure according to the machining speed.

CONSTITUTION: In a machining condition setting device 30, an arithmetic condition input part 31 extracts and inputs material and thickness of a work and a speed set value for every machining time from an NC program inputted to an NC device. A data file selection part 32 retrieves a data file 36 classified for every work condition from the material and thickness of the work inputted and selects a specified data file. A machining condition arithmetic part 33 applies the inputted machining speed and calculates the optimum power, pulse frequency, duty ratio and assisting gas pressure for one machining speed by using the selected data file. A condition setting device 34 for a laser beam controller sets values calculated on the power, pulse frequency and duty ratio of a laser beam. A condition setting device 35 for a gas controller sets automatically the assisting gas pressure.