An apparatus in which ultrasonic vibrations or acoustic emissions are detected includes a rotating shaft 21 having a dressing wheel 31 mounted on one end, and a detection liquid supply unit 41 disposed at the end face of the wheel and/or shaft so as to form a gap S therebetween and to supply detection liquid toward the rotational center of the wheel to form a detection liquid membrane in the gap. An AE sensor 43 is provided integrally with the detection liquid supply unit and detects the ultrasonic vibrations or acoustic emissions of the wheel and/or shaft through the detection liquid membrane formed in the gap.