A background pattern print image-forming method and device are provided. A plurality of kinds of unit background pattern image data formed of a dot matrix which defines a group of image-forming dots representative of a unit printing portion of a background pattern to be printed on a printing object are stored in a memory device. One kind of unit background pattern image data from the plurality of kinds of unit background pattern image data stored in the memory device is selected as a unit background pattern tile. The selected unit background pattern tile is read from the memory device, and modified to thereby form a different kind of unit background pattern image data from the selected one kind of unit background pattern image data. The different kind of unit background pattern image data is arranged in a manner such that the background pattern is formed, to thereby form background pattern print image data.