PURPOSE: To detect the presence of voids, by arranging a microphone adjacent to a part of a roll where grinding is being carried out when the surface of a coated layer of a rubber or plastic-coated roll is ground, so that abnormal sounds during the grinding are received by it and are electrically amplified to be detected.

CONSTITUTION: A rubber or plastic-coated roll 1 is rotated, and the microphone 3 is positioned adjacent to the part of the roll 1 where grinding is being carried out, so that the sounds during the grinding (grinding sounds) are received by a microphone 3 at all times, and are converted to an electric current corresponding to the sounds to be recorded in a recorder connected to the microphone 3 in an electrically amplified form. Therefore the abnormal sounds different from the normal sound that will be generated when the grinding stone has passed the part of the surface of the roll 1 in which there are voids are all recorded in the recorder, so that the number of the voids in the coated layer can be precisely detected.