To surely execute a binding process even when sheets of different sizes are included in a bundle of sheets by retrieving a minimum sheet size among stacked sheets when the stapling process is executed on at least two parts of the bundle of sheets, and controlling the stapling process of the bundle of sheets on the basis of a result of the retrieving.

Sheets to be conveyed by a slip roller 7 are kept in a standby state in a condition that a shutter 9 is closed, and the information on sizes of all sheets and whether the stapling is to be executed or not, is transmitted to a CPU of a staple stacker 1, when the printing is started in a printer in this standby state. Whereby the information on sizes of sheets are completely stored in a memory, and whether the size of sheets of a bundle to be stapled is one kind or a plurality of kinds, is judged. When a plurality of kinds of sizes are included, a minimum sheet size is retrieved from the information on sizes of the sheets of the bundle to be stapled, then a stapling position corresponding to the minimum size is determined, and then the operation of a stapling unit 11 is controlled to execute the binding process.