To perform suitable machining to a workpiece such as a magnetic core block with high dimensional accuracy, dispensing with an external setup process.

With the feed operation of a wire 7 stopped, the wire 7 is positioned to a cutting position of the magnetic core block 100, and the wire 7 is brought into contact with the cutting position of the magnetic core block 100. With the wire 7 in contact with the cutting position of the magnetic core block 100, the feed operation of the wire 7 is performed for a designated time to thereby forming a wire holding groove 10 in the cutting position of the magnetic core block 100. With the wire 7 held in the wire holding groove 10, actual cutting is started.