To reinforce joining sections by surrounding a space by the joining sections, in which a large number of beltlike bodies and adjacent beltlike bodies are joined alternately at every two, and arranging and knitting strands, on which the peripheries of the beltlike bodies are covered with a thermoplastic synthetic resin, at right angles.

A reinforcing body is constituted of joining sections 2, in which a large number of beltlike bodies 1 and adjacent beltlike bodies 1 are connected alternately at every two, and spaces 3 surrounded by the joining sections 2. In the beltlike bodies 1, strands, in which a singular number or a plurality of synthetic-fiber high-strength yarn are used as core materials and the peripheries of the yarn are covered with a thermoplastic synthetic resin, are arranged and knitted at right angles in the longitudinal directions and cross directions of the beltlike bodies 1.