A hollow structural component which includes an integral structure, partition walls intersecting each other within the integral structure, and a plurality of independent hollow portions formed as specified by the partition walls so as to be arranged within the integral structure. The integral structure is formed into one piece by one-shot molding or by one injection molding die which is caused to slide against each other between its stationary side and movable side for post-forming, whereby weight reduction of the hollow structural component, reduction in the amount of resin used, reduction in the number of parts involved, and improvement in the workability for assembling may be achieved with consequent cost reduction.