PURPOSE: To detect the insertion of two pieces with high accuracy and good efficiency by a method in which on the basis of signals from a sensor, the reference of a paper thickness is prepared, when the thickness of paper to be detected exceeds a fixed value, the paper is discharged by two-piece insertion detecting operations, and a control signal is sent out to a given position of printer according to the reference thickness.

CONSTITUTION: The first paper P is sent out to a thickness sensor 1, and a detecting timing signal (k) is sent out from CPU 11 to a delivery timing, where the actual thickness of the paper P is detected in the form of an analog value. In the same way, the thickness of the second paper P is measured, the output signal (f) is put in a comparison circuit 15 and compared with a reference value. When the output signal (f) is above a fixed value over the reference value, it is regarded as two-piece insertion and an abnormal signal (h) is sent out to CPU 11. On the basis of the signal (h), the CPU 11 sends out a command (h) to a solenoid 17 for discharging papers, and the second paper P is removed by the solenoid 17. Since the two-piece insertion can be detected with high accuracy and good efficiency, the efficiency of printing operations can be raised.