PURPOSE: To prevent a shaping shortage and over cutting by detecting the change in the rotary number of a rotary blade electrically and by confirming the shaping completion via the detecting signal thereof.

CONSTITUTION: The rotary blade 4 having the curved part which is conformed to the tip shape of electrode tips E1, E2 is supported by a holder 3 and the holder 3 is driven with rotation by air motor 2 via a rotation transmitting mechanism 5 as well. In this case, the load of the air motor 2 driving the rotary blade 4 is heavy at the beginning and low in the number of rotations. With the development of the tip shaping the load of the motor 2 is reduced and when the tip shape of tips E1, E2 reaches upto the prescribed shape the number of revolutions of the motor 2 becomes constant. The detector 9 of the number of revolution detects the number of revolutions thereof electrically and completes the shaping work. The shaping shortage and over cutting of the tip E1 and E2 can thus be prevented.