PURPOSE: To obtain a formed body, which has flexibility and excellent dynamic characteristics, by a method wherein a specified number of composite fiber units fulfilling specified conditions are bound up in a bundle by winding with the same thermoplastic resin as the thermoplastic resin, with which the units are impregnated and covered.

CONSTITUTION: From 2 to 1,600 composite fiber units 1, each of which is produced by impregnating and covering a plurality of reinforcing fiber monofilaments 3 with thermoplastic resin 2 so as to satisfy a conditional expression E.S.m.a2≤50, are bound into a bundle by winding with wound fibers 16 made of thermoplastic resin fiber, which is the substantially same as the thermoplastic resin fiber impregnating and covering the filament. In this case, E represents the elongation elasticity modulus (kfg/mm2) of reinforcing fiber monofilament 3, S represents the sectional area (mm2) of the reinforcing fiber monofilament 3, (m) represents the number (-) of reinforcing fiber monofilaments in a composite fiber unit 1 and 2a represents the minor axis of the section of the composite fiber unit 1. The wound and bundled thermoplastic fibers are locally melted so as to be fused to the composite fiber units 1 in order to bundle and fix the units 1.