In a high voltage P-channel MOS transistor formed on a silicon-on-insulator substrate, a P+-type source region, an N-type body region and an N+-body contact diffusion region are surrounded by a P+-type drain region and a P-type drift region. During and after assembly, the elastic engagement pieces elastically abut at least an inner surface of a tapered hole in a main shaft joined to the tool holder and deform slightly in a radial direction. The Hall detector detects magnetic flux variation when the rotor rotates and generates a first detection signal and a second detection signal. Also, the operating speed is adjusted in response to the determination. The first DC lead is electrically coupled to the EAM-side capacitor electrode and configured to be coupled to a first DC potential.