To provide the subject method and device capable of restraining the amount of curved cutting for improving the machining accuracy, and of a sure cutting work while improving the service life thereof.

A detector detects the amount of curved cutting during operation, then an arithmetic processing unit 15 compares a limit value of curved cutting inputted from an input means and stored in a memory 13 with a value detected by the detector 9, and stops the operation when the detected value is larger than the limit value. When the detected value is smaller than it but larger than a specified set value, on the other hand, a saw speed controller 19 reduces only the cutting speed or both the cutting speed and the saw speed to restrain the amount of curved cutting for a sure cutting work.