This invention provides a reinforcing woven fabric using as weaving threads flat and substantially nontwisted multifilaments of reinforcing fiber having less than 0.5 wt. % of sizing agent provided thereto and a binding property of 400-800 mm in hook drop value, and a method and an apparatus for manufacturing a reinforcing woven fabric by weaving the multifilaments of reinforcing fiber as warps and wefts. A reinforcing woven fabric having a small thickness and an extremely high covering factor and capable of indicating a high strength can be obtained at a low cost as a substrate for reinforcing composite materials.