PURPOSE: To maintain the accuracy of discrimination even when status relating to the detection is deviated from reference conditions (regular conditions) by matching them based on the correcting value of a sought feature quantity.

CONSTITUTION: Respective detected values Di are found by magnetic resistance elements Mj of a magneto-resistance element array 2, mean value Dm of the respective detected values Di is found by Dm=(ΣDi)/N and here, N is the total number of the respective detected values Di, 10 in this case. A correcting coeffi cient (a) is found by a=Km/Dm and here, Km is the mean value of respective reference detected values Ki. Then, respective correcting values aDi are sought, an operation relating to pattern matching as a subroutine is executed based on the respective correcting values aDi, and the fluctuation of the respective detected values of a feature quantity is compensated (regulated). Thus, required discriminating accuracy can be maintained corresponding to the fluctuation of the detected value of the feature quantity.