1. An image processing apparatus for inputting a document image and embedding watermark information by controlling a line spacing between character strings in the document image, comprising: input unit for inputting a document image as image data; image reduction unit for, letting W0 be the number of pixels in a first direction of the input document image and H0 be the number of pixels in a second direction perpendicular to the first direction, generating, from the input image data, reduced image data whose number of pixels in the first direction is W1 (1<W1<W0); determination unit for determining reduced objects corresponding to character strings in the image data by scanning the reduced image data generated by said image reduction unit and comparing sizes of reduced objects in the reduced image data with a reference size designated in advance; specifying unit for specifying a position of each character string in the image data on the basis of positions of the reduced objects in the reduced image data, which are determined by said determination unit as the reduced objects corresponding to the character strings; and embedding unit for embedding the watermark information in the image data by moving positions of a plurality of character strings in the image data, which are specified by said specifying unit, on the basis of a preset line spacing correspondence relationship.