PURPOSE: To detect maximum frequency and to detect positional shear precisely by executing the thick or thin processing of a pattern to be matched, finding out the centroid coordinates of the remaining part and comparing said centroid coordinates with reference centroid coordinates which have been previously registered.

CONSTITUTION: A picture inputted from a picture input device 10 of a pattern positional shear detector is accumulated in the 1st storage device 20, a thickening processing circuit 30 and a thinning processing circuit 40 executes the thickening and thinning processing of the accumulated picture respectively. The centroid coordinates of plural partial patterns surrounded by a background part including in the picture processed by the circuits 30, 40 are detected by a centroid detecting circuit 50. A prescribed processing for a picture previously inputted to a reference file 80 is executed and the centroid coordinates are found out and registered. The reference centroil coordinates registered in a file 80 are compared with the centroid coordinate of an input picture found out by the circuit 50 by a rotational shear detecting circuit 60 and a parallel shear detecting circuit 70 to detect the positional shear precisely.