A system for fastening a cover to a body of an explosion-proof enclosure using a plurality of fastening devices. If a permission for a notice of an incoming call is set, a display device displays information of \u201cunder an answer suspension state\u201d indicating that an answer to the relative incoming call is being suspended. The second retainer includes a flexible arm having a free end portion which is flexible in a first direction parallel with the panel body and a second direction perpendicular to the panel body. To accomplish vibrational stimulation, the stimulation means is brought to vibrate at a frequency of 1-5000 Hz within the body cavity.