Whether the inside of a beam path is a part (PT) or the outside is the part at a corner (CN) is determined from an offset direction that indicates whether the laser beam (RB) be offset toward the right side or the left side in the direction of progress included in the motion definition sentence and from the bending direction at the corner in the beam path (bi, bi+1). When the inside of the corner is the part, an escape path (EP) that has been set in advance is inserted in the corner portion of the beam path to prepare an NC PERT programm for laser machining. When the outside of the corner is the part, an NC PERT program for laser machining is prepared so that similarly set machining conditions are obtained within a preset machining condition changing section (QP, PR).