PURPOSE: To execute a working having a uniform quality without requiring the knowledge and the skill of working by constituting a titled machine so that a cutting speed is obtained based on a laser output determined in accordance with a material quality and the plate thickness of a work, which are inputted, and the material quality and the plate thickness.

CONSTITUTION: A laser output LO being suitable for a working having a material quality MT and a plate thickness TH is determined by retrieving a laser output table in a memory 15 in accordance with the material quality MT and the plate thickness TH of the work, which are inputted from an input means 5. Subsequently, this machine is constituted so that a cutting speed FR being suitable for the laser output LO, the material quality MT and the plate thickness TH is determined by retrieving a cutting speed table in a memory 19, based on the determined laser output LO and the inputted material quality MT and the plate thickness TH. Accordingly, the optimum cutting speed FR is determined immediately by only inputting the material quality MT and the plate thickness TH of the work.