To finish a warp in a thin plate-like form without bending it excessively by using a multi-filament yarn with a flat cross-section made up of a high strength fiber material as the warp and laminating a plurality of pieces of the warp in individually different directions to form a reinforcing material.

This composite structural material comprises a plurality of reinforcing materials 10, 10... made up of a fabric texture impregnated with a matrix material 21 so that the entire structural material is of a thin plate-like shape. A warp 11 is constructed of a multi-filament yarn, made up of high strength fiber, which is composed of a required number of filaments bundled in parallel so that the warp 11 is about 2-5 mm wide and about 0.1-0.4 mm thick with a flat cross-section. The reinforcing materials 10, 10... are laminated in such a manner that the directions of the individual warps 11, 11... are alternately orthogonal with each other, and then are impregnated with the matrix material 21 into a set thin plate-like form. Besides, the reinforcing materials 10, 10... are laminated in such a way that their directions are alternately set, and in accordance with the order of codes (1), (2)... from the bottom.