An image detecting method is provided that permits the specific image information to be present within image data of a document and is capable of detecting the specific image information within a relatively short period of time and accurately, with little effect of interference with the image data of the document and without regard to an angle at which the document is placed. The specific pattern image comprises a plurality of dot images Sa and Sb arranged in a straight line, and by positioning one of the plurality of dot images Sa and Sb at the center O of a plurality of concentric circles of different diameters and detecting that other dot images are on the plurality of concentric circles R1 to R4 and are on a straight line L running through the center O of the concentric circles, the pattern image is detected.