1. A non-transitory computer-readable storage medium storing a program for a computer used for embedding information in a background region of a document image by using a background texture pattern including a plurality of dots, the program causing the computer to execute a process comprising: combining and arraying a plurality of background texture patterns in which a plurality of dots are arranged in such a manner that different features are extracted respectively from regions not having a dot, and thereby generating a sequence of background texture patterns expressing information to be embedded; and embedding the sequence of the background texture patterns in the background region of the document image, and generating a synthesized image.