Method of detecting foreign matter and apparatus therefor

An apparatus to detect the presence or absence of foreign matter in a product, the apparatus comprising:

• a reaction signal generating circuit outputting a reaction signal associated with the product by detecting a change in pattern of distribution of magnetic fluxes when the product is passed through an alternating magnetic field;
• a determining circuit comparing a value of the reaction signal at a predetermined phase point with a threshold value to determine the presence or absence of the foreign matter in the product;
• a sensitivity adjusting unit adjusting a sensitivity of the reaction signal generating circuit so that a value of a first test reaction signal at the predetermined phase point, which is obtained when the product comprising a foreign matter of a minimum size to be detected is passed through the alternating magnetic field, attains a desired level relative to the threshold value;
• and
• a first optimum phase setting unit setting as the predetermined phase point a phase at which a value of a second test reaction signal obtained when the product having no foreign matter is passed through the alternating magnetic field is minimal.