1. An image processing apparatus for inputting a document image and extracting watermark information embedded in the document image on the basis of a line spacing between character strings in the document image, comprising: an image reading apparatus for inputting a document image as image data; an image reduction unit for generating, from the input image data, reduced image data, wherein the reduced image data is generated by reducing the input image data in a first direction only, or is generated by reducing the input image data in the first direction and in a second direction that has a predetermined relationship with respect to the first direction, wherein the reduction ratio in the first direction is higher than the reduction ratio in the second direction; a detection unit for scanning the reduced image data generated by the image reduction unit in the second direction and detecting a length of a blank region in the second direction as line spacing information; and an extraction unit for extracting watermark information embedded in the document image on the basis of the line spacing information detected by the detection unit.