A core image acquiring unit is mounted at a position where the core image acquiring unit looks down at a workpiece on a table, such as an upper guide unit of a wire electric discharge machine, the core machining is performed, while fixing the core using a core fixing function, at a stage in which machining is completed to the end point of the machining path of cutting the core, the core and the workpiece are picked up by the core image acquiring unit, from the captured image, it is determined whether a machining groove has a constant width, or adhesion and deposition of machining debris are present in the machining groove, or the core is fixed by a difference in states of the surface light between the core and the workpiece. When the core is not fixed, since the machining is stopped, the fixing of the core is checked, and the machining is advanced, continuous machining can be safely performed. Further, when the core is dropped without being fixed, since the machining is stopped, it is possible to prevent brea
