Precast structure, and construction method for underground structures

A segment 3 is made from concrete and has an external shape substantially the same as a segment used in shield tunneling. That is, the segment 3 comprises arc-shaped curved faces that face each other and side faces to couple the curved faces to each other. That is, a precast structure 1 comprising the segment 3 has a ring shape. A ring-shaped precast structure can be formed by connecting a plurality of segments 3 in the circumferential direction. Prestressing is applied by a tensioning member that spans the entire length of the connected segments 3 in the longitudinal direction. In the same manner, prestressing is applied by a tensioning member in the circumferential direction of the connected segments 3.