PURPOSE: To prevent the mistake and stop of the stapling by detecting the thickness of a form bundle discharged onto a bin and changing over and controlling the execution or nonfulfilment of staple operation by the result of the detection.

CONSTITUTION: When paper is discharged from a copier, and a fixed number of forms are stacked onto each bin 4 through a distributing roller and aligned completely, a clamper (s clamping moving means) 19 is operated by a solenoid 25, and a form bundle 7 is held between an upper clamping piece 23 and a lower clamping piece 24. the thickness of the form bundle 7 is detected by a holding sensor 21 or a rotary type potentiometer 22 on the holding operation. A control means selects the kind of a binding needle adapted by the thickness of the form bundle 7 detected, decides the execution/nonfulfilment of staple operation, and drives a motor 26 and leads out the form bundle 7 to the staple unit side when it decides execution.