PURPOSE: To prevent sheet from breakage to be caused by such an additional treatment as punching, stamping, etc. by exhausting the sheet when skewing is sensed thereon in such a way that the sheet passes through the punching and stamping mechanism or the stamping mechanism while it is off service.

CONSTITUTION: If sensor 480A of a punch control unit 400 is sensing a sheet existing while another sensor 480B is not sensing, it is judged that there is a skew, which shall be informed to a control board unit to shut off the current to solenoid 428. Thus the sheet is exhausted without being punched. If the sensor 480A is not sensing a sheet existing while the other one 480B is sensing, on the other hand, it is judged that there is a skew, and the sheet is exhausted in the same manner as above. When the signal of sheet skewing is sent to the control board unit, a non-stamp command data is sent to a stamp control unit, and the mode data is updated into non-stamp to allow the sheet to automatically pass the stamping mechanism without service. Treatment in the stamp control unit is the same as that in the punch control unit, i.e. the sheet is passed through only the stamping mechanism without service to be exhausted.