A method for manufacturing wire harnesses (W/H) of a plurality of types of wire harnesses to which each a product number is allocated, the method for manufacturing comprising: acquiring first manufacturing specification information representing the specification of the wire harnesses (W/H) for each product number; extracting, as a primary common component, all components commonly provided in the wire harnesses (W/H) of all the product numbers based on the first manufacturing specification information; determining, as a basic part, a sub-harness of the wire harnesses (W/H) to be manufactured formed of only the primary common component; classifying, as an optional component (Po), the rest of the primary common component among all the components that form the wire harnesses (W/H) of all the product numbers; acquiring information indicating a predicted production quantity (Np) for each product number and a usage rate (Ur) of each component; extracting, as a secondary common component, a plurality of component groups commonly provided in at least a plurality of product numbers among the optional components (Po) of which the usage rate (Ur) is equal to or higher than a predetermined value among the wire harnesses (W/H) of the plurality of product numbers extracted based on the predicted production quantity; determining, as additional parts, configurations of sub-harnesses of the wire harnesses (W/H) to be manufactured formed of only the extracted secondary common component; classifying, as an additional component (Pa), each component of the rest of secondary common components among the optional components (Po); determining, as second manufacturing specification information, a configuration of the wire harness (W/H) of each product number based on a combination of the sub-harness of the basic part, the sub-harness of the additional part, and the additional component (Pa); and manufacturing the whole wire harness (W/H) of each product number based on the second manufacturing specification information with a combination of the sub-harness of the basic part, the sub-harness of the additional part and the additional components (Pa). acquiring first manufacturing specification information representing the specification of the wire harnesses (W/H) for each product number; extracting, as a primary common component, all components commonly provided in the wire harnesses (W/H) of all the product numbers based on the first manufacturing specification information; determining, as a basic part, a sub-harness of the wire harnesses (W/H) to be manufactured formed of only the primary common component; classifying, as an optional component (Po), the rest of the primary common component among all the components that form the wire harnesses (W/H) of all the product numbers; acquiring information indicating a predicted production quantity (Np) for each product number and a usage rate (Ur) of each component; extracting, as a secondary common component, a plurality of component groups commonly provided in at least a plurality of product numbers among the optional components (Po) of which the usage rate (Ur) is equal to or higher than a predetermined value among the wire harnesses (W/H) of the plurality of product numbers extracted based on the predicted production quantity; determining, as additional parts, configurations of sub-harnesses of the wire harnesses (W/H) to be manufactured formed of only the extracted secondary common component; classifying, as an additional component (Pa), each component of the rest of secondary common components among the optional components (Po); determining, as second manufacturing specification information, a configuration of the wire harness (W/H) of each product number based on a combination of the sub-harness of the basic part, the sub-harness of the additional part, and the additional component (Pa); and manufacturing the whole wire harness (W/H) of each product number based on the second manufacturing specification information with a combination of the sub-harness of the basic part, the sub-harness of the additional part and the additional components (Pa).