Abstract: An underground conduit-forming structure made of concrete for easily forming a protection conduit line for laying underground cables, e.g., power cables, communication cables, etc. For snug coupling between the leading and trailing structures, each structure is provided with a flat bottom, a plurality of through-holes for cables, a receiving surface and a corresponding mating surface, a protuberance and a corresponding fitting hole, operation holes for the coupling operation, joint metals, and a reinforcing bar secured by welding some of these members. A seal member encompassing the through-holes keeps them water-tight when a series of structures are sequentially coupled to form the protection conduit line.