An apparatus for detecting contact between a contact detection member and a rotating body such as a grinding wheel rotably carried on a wheel head which is movable toward the contact detection member in a grinding machine for example. The apparatus comprises an AE sensor, a rectangular pulse generator, a sampling circuit and a processing device such as a microcomputer. The AE sensor generates an electric signal in response to vibration of the contact detection member. The rectangular pulse generator outputs a rectangular pulse whose duration corresponds to that of vibration. The sampling circuit delivers to the processing device an output indicating the duration of the rectangular pulse. The processing device processes the output delivered thereto so as to decide the occurrence of an actual contact between the rotating body and the contact detection member.