1. A product quality control system for controlling the quality of a product manufactured through a plurality of manufacturing processes and at least one inspection process, comprising: a manufacturing sequence information management apparatus for storing and controlling the information on the order in which selected one of a material, a part and a product in progress used for manufacturing the product is input into each manufacturing process; a manufacturing management apparatus for storing selected one of the product, the material, the part and the product in progress with corresponding unique individual identification information and giving an instruction to manufacture the product based on the individual identification information, the sequence information supplied from the manufacturing sequence information management apparatus and a product manufacturing plan; and a causation analysis apparatus of quality variation for analyzing the cause of product quality variation based on the manufacturing history data collected by the manufacturing history data collection apparatus associated with each manufacturing process and corresponding to the individual identification information on the one hand and the product quality history data of the product corresponding to the individual identification information and measured at the inspection process on the other hand; wherein the causation analysis apparatus of quality variation includes a correlation analysis module of quality variation for calculating by collation the statistical correlation magnitude between the quality history data and the manufacturing history data using the individual identification information and automatically extracting the candidates for at least one manufacturing process likely to cause the product quality variation based on the statistical correlation magnitude, and a causation analysis module of quality variation for calculating by collation the mutual statistical correlation magnitude between the manufacturing history data themselves using the individual identification information, and based on the statistical correlation magnitude and the sequence information thus obtained, determining the connecting structure information between the manufacturing processes thereby to automatically determine and extract the manufacturing process causing the product quality variation from the candidates for the product quality variation causing processes.