A model producing means for modeling a chip model at an edge of a solid model of a nozzle through a predetermined space in a three-dimensional virtual space, and a trace machining propriety judging means for watching a state of an interference between the chip model and the solid model of a workpiece at the time of simulation of laser beam machining program and outputting a predetermined signal when releasing the state of the interference are provided. If an inconvenient instruction wherein the nozzle and the workpiece do not face each other is included in the laser beam machining program, the chip model departs from the solid model of the workpiece and the trace machining propriety judging means outputs a predetermined signal, through which program error can be properly detected with no visual observation of an operator.