1. A method of selecting an inspection-required product lot for a sampling inspection during the production process to minimize the expectation value of the number of the investigation steps for a defect-causing material lot at the time of defect occurrence of the product placed on the market, comprising the steps of: determining the amount of the product produced during a scheduled future period based on the received order information; calculating from design information the amount of the materials required for the product scheduled for production; preparing production plan information in which the material lots used for each material are allotted to each lot of product scheduled for production based on material stock information and in accordance with a predetermined allotment rule; accessing the information inputted by a user including the number of inspectable product lots indicating the number of the product lots to be produced during the scheduled production period, a parameter for arithmetic operation and optimization conditions constituting a criterion; and assuming all conceivable combinations of the product lots for the number of the inspection-required product lots of all the product lots; and further, with regard to each of the combinations, extracting, as investigation-unrequired material lots, the material lots allotted to the inspection-required product lot: totalizing, for each product lot scheduled for production, the number of the allotted material lots other than the investigation-unrequired material lots specified in the preceding step; calculating statistical values, such as statistical primary optimization value and statistical secondary optimization value, of the number of the investigation-required material lots totalized in the preceding step; and selecting a combination of the product lots minimizing the statistical primary optimization value for the number of the investigation-required material lots for each product lot, as calculated for each of the combinations; in the case where a plurality of corresponding combinations of the product lots are selected in the preceding step for selecting a combination of the product lots minimizing the statistical primary optimization value, further selecting a corresponding combinations of the product lots for a combination minimizing the statistical secondary optimization value; and in the case where only one combination of the product lots minimizing the statistical values is selected in the preceding step, selecting the corresponding combination of the product lots as an inspection-required product lot.