An apparatus for monitoring an inside of a manhole in which the inside of the manhole is monitored using a simple communication and power supply unit is closed. The apparatus comprises power cables installed inside a manhole, a connecting unit connecting the corresponding power cables, a monitoring communication control unit colleting maintenance information about the corresponding power cables and performing signal processing of the maintenance information, a power supply unit supplying power directly from the power cable or the connecting unit to the monitoring communication control unit, a signal injection unit digitalizing the maintenance information collected by the monitoring communication control unit and injecting the digitalized information into the power cable to transmit the maintenance information to a next manhole, and a signal receiver installed in the power cable or the connecting unit and receiving maintenance information transmitted via the power cable to input the received maintenance information to the monitoring communication control unit. Accordingly, a device and a communication line need not to be installed outside the manhole and a power line need not to be drawn out from the outskirts of a city and the inside of the manhole can be simply and conveniently monitored.