To solve problems of a conventional edge extract processing that has required an increased memory capacity needed for the processing and an increased processing quantity because the conventional edge extract processing extracting an edge from image data employs multi-value image data.

The edge extract processing of this invention retrieves an edge of binary image data by using an edge detection pattern consisting of a couple of pixel arrays each comprising a prescribed number of pixels having the same gray level, that are opposed to each other having a center pixel in-between and where the gray level of the pixels included in one pixel array differs from the gray level of the pixels included in the other pixel array so as to solve the problems above.