If a threshold discrimination is performed with variable Z=0 using discriminant analysis, that is useless unless know-how is accumulated through visual judgment and actual operation. A discriminant function is computed using a plurality of parameters which make pass/fail judgment factors and the results of that pass/fail judgment. With respect to the discriminant function, a histogram is generated for pass category and for fail category. Then, a threshold is determined based on the standard deviation in the individual categories so that an intended rate of flowout and rate of overcontrol will be obtained. The acceptability of pass/fail judgment objects is judged based on the threshold. Thus, the rate of flowout and the rate of overcontrol can be controlled as intended. Further, high-performance pass/fail judgment can be implemented without accumulating know-how.