PROBLEM TO BE SOLVED: To provide a segment for which a connection structure is strong and connection work is easy.SOLUTION: A composite segment 1 includes a rectangular frame body 2 (a main girder 5 and a joint plate 6), a skin plate 3 and a vertical rib 4. The main girder 5 comprises a main girder body 10 and connection parts 20 positioned at both ends thereof. The main girder body 10 is formed in an U shape including an outer side flange 11, an inner side flange 12, and a web 13. The connection part 20 comprises a connection step part 30 and a connection projection part 40. The connection step part 30 includes a step side connection plate 31 and a step side partition plate 32, and the connection plate 31 is provided with an engaging projection 33. The connection projection part 40 includes a projection side connection plate 41 and a projection side partition plate 42, and an engaging hole 43 is formed through the connection plate 41. Then, by relatively moving segments connected in a tunnel circumferential direction with each other in a tunnel axial direction while bringing them into surface contact on the joint plates 6 of each other, the engaging projection 33 of one segment is fitted to the engaging hole 43 of the other segment.
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