PURPOSE: To invariably perform stable machining by feeding the new set amplitude value outputted from an amplitude adding circuit to a high-frequency oscillator, and increasing or decreasing the amplitude after the load applied to a work.

CONSTITUTION: When the load applied to the tip of a tool 14 is detected by a load detecting means represented by a load detector 62, it is converted into the amplitude increased/decreased value suitable for the load at the tip of the tool 14 by a load/amplitude converting circuit 64. The amplitude increased/ decreased value outputted from the load/amplitude converting circuit 64 is inputted to an amplitude adding circuit 66, the amplitude increased/decreased value is added to the amplitude set value at the time of no load, and the set amplitude of a high-frequency oscillator 58 is changed.