PURPOSE: To reduce similarity dispersion caused by the position of an object (graphic) inside a visual field in the case of pattern recognition using a reduced image by additionally providing a third image memory and a product sum operation unit.

CONSTITUTION: A section 17 to be newly provided is equipped with a known image memory 18 for storing the data of a reduced reference image. On the other hand, a product sum operation unit 19 is provided for calculating the maximum similarity between the reduced image in a memory 11 and the reduced reference image in the memory 18 together with a processor 16 while using data in the image memories 18 and 11 and being paired with the image memory 18. Thus, the extended section 17 is provided with a pair of the memory 18 and the product sum operation unit 19, when any known reduced image at a prescribed position is preserved in the known image memory 18, correlation coefficient are provided through correlative arithmetic together for the two images of the known image in the memory 12 and the known image in the memory 18, and the maximum correlation coefficient is provided from those coefficients. Thus, satisfactory pattern matching can be performed.