Abstract: An image input section enters a document image including a bar code, and converts the document image into a binary image. An image memory section memorizes the binary image supplied from the image input section. A mask judging section performs a scanning operation using an M.times.M pixel window for detecting an alternate pattern from the binary image supplied from the image input section. A small pattern removing section removes small patterns from a binary image sent from the mask judging section. An expanding section expands a pattern involved in a binary image by an amount equivalent to k1 pixels when this binary image is sent from the small pattern removing section. A contracting section contracts a pattern involved in a binary image by an amount equivalent to k2 pixels when this binary image is sent from the expanding section.