1. A pass/fail judgment device which takes the form of pass/fail objects as a pass/fail judgment factor, and which is used to detect a defective unit in product inspection, comprising: a discriminant function computing unit for computing discriminant functions which give variables used to separate frequency distributions of a pass category and a fail category from a plurality of pieces of parameter information which make pass/fail judgment factors and pass/fail judgment result information, wherein each of the frequency distributions of the pass category and the fail category has a shape of a normal distribution; a statistical parameter computing unit for computing a center of distribution and distribution parameters that vary in accordance with a breadth of a distribution for said variables with respect to either or both of said pass category and said fail category; a threshold determining unit for determining a threshold for providing a pass/fail judgment based on the value of a variable and giving a specific distribution probability that is based on at least one of a rate of flowout in the fail category, which represents a number of pass/fail judgment objects that are actually in the fail category and that are judged as being passed, and a rate of overcontrol in the pass category, which represents a number of pass/fail judgment objects that are actually in the pass category and that are judged as being failed, relative to said center of distribution and said distribution parameters; a parameter information acquiring unit for acquiring a plurality of pieces of parameter information on one or more pass/fail judgment objects according to data of a normal distribution; and a pass/fail judging unit for comparing the value of variables obtained by substituting the parameter information into said discriminant function with said threshold and for thereby providing a pass/fail judgment for the one or more pass/fail judgment objects; wherein the overcontrol and flowout are separated having the normal distribution, wherein the overcontrol and flowout are judged based on the data of the normal distribution.