It is of prime importance to conduct maintenance of electric power transmission lines such as underground cables and overhead lines regularly, in order to secure a stable and reliable supply of electric power. Various devices that require electric power are needed for maintenance work. However, it can be difficult to position an adequate power source within a power cable manhole, or in the vicinity of an overhead power line. In the latter case, a combination of solar and storage batteries is frequently used, especially in mountainous areas; large storage batteries are needed to cover continual rainy days or night operation. In the case of manholes, since solar batteries cannot be used, storage batteries need frequent re-charging, during which time the manhole covers must be kept open. If the entrance to the manhole is on a road, traffic is obstructed. Under these circumstances, development of a compact on-site electric power source, which does not need re-charging, is desirable.