A sheet processing device includes a carry-in path, and a retreat path that branches off from the carry-in path. A bonding device is disposed at a merging point between the carry-in path and retreat path. An adhesive-applied position of a preceding sheet is retreated to the retreat path when a next sheet is carried in to the stacker section, and then the next sheet is moved to a bonding position. The above operations are sequentially repeated to form a sheet bundle. Thus, the sheet bundle can be formed by bonding the sheets using an adhesive. It is possible to reduce stress to be applied to the sheet, and to comparatively reduce movement of the sheet applied with the adhesive, thereby preventing the sheets from being bonded to each other at a position other than a predetermined position.