<p>Detection and transmission of an ultrasonic signal with high S/N ratio is realized without requiring any component produced with high machining precision and without requiring the components to be assembled with high precision. A dresser head (41) is provided with an ultrasonic sensor (21) for detecting ultrasonic sound, a signal conversion transmission circuit (23) for converting the detection signal from the ultrasonic sensor (21) into a digital signal and radio-modulating the digital signal, and a signal transmitting antenna (26) to which the radio-modulated signal is applied. Near the dresser head (41), a signal receiving antenna (34) for receiving the signal radiated from the signal transmitting antenna (26) and a signal receiving head (35) for demodulating the received signal are provided. The demodulated signal is outputted as a digital or analog signal of a predetermined signal form through a control section (31).</p>
