PURPOSE: To prevent erroneous decision due to paper slipping from happening by extending picture elements at the edge parts of a sample paper image to optional directions which are vertical and lateral.

CONSTITUTION: Image data on a sample paper is fetched by a sensor, and is written in a sample paper memory 20b. Then, zobel operation for this is performed by a zobel differentiating circuit 20c. When specified picture elements, e.g. picture elements (I, J), which are determined by this zobel operation are at edge parts, by extending the edge parts further, the periphery of, e.g. the top and bottom, and right and left, etc., is written in an extended edge of picture elements of edge ±1 and an edge memory 20d. At the edge region in the edge memory 20d, a setting value circuit 20e is equipped, in order to make the allowable value, which is a decision level, more liberal. Thus, a difference portion between the image data of the sample paper and image data on an input paper is taken (difference circuit 20f), and this difference level is compare-decided by a comparison unit 20g, with a set value in the setting value circuit 20e for which the only edge region is made liberal. Then, NG picture element detection is performed by deciding difference portions by an original setting level for areas other than edge parts.