Abstract: Maintenance information is collected by a sensor on an electric power cable installed in a manhole. The information is transmitted to a radio transmitter which is positioned in a vicinity of a bottom plane of an inner lid in an opening of the manhole. The information is transmitted by the radio transmitter, and received by a receiving antenna mounted on a bottom plane of the inner lid. On a top plane of the inner lid, an electrode, a connecting lead or a transmitting antenna is provided to be connected to the receiving antenna by an impedance matching circuit. The information is supplied to an outer lid in the opening of the manhole by the opening of the manhole by the electrode using a spatial coupling capacitance relative to the outer lid, or the connecting lead directly connected to the outer lid. Thus, the information is transmitted as a radio signal by the outer lid functioning as an antenna. Otherwise, the information is radiated below the outer lid by the transmitting antenna.