PURPOSE:To enable the direct and non-contact monitoring of the operation condition of a paper feeding belt by detecting the mark of the belt non-contactly to count its output signal, and comparing the output signal to the preset value to obtain the deviation, and alarming when the deviation exceeds the specified value. CONSTITUTION:The mark 6 is adhered to a paper feeding belt 1, and a mark sensor 7 is provided at the through position spacedly facing to the mark. As the mark sensor 7, a mark sensor for detecting the mark 6 non-contactly and for generating the electric signal is appropriately selected. When the electric signal generated by the mark sensor 7 is sent to a computer alarm 8, the computer alarm 8 in which a counter for counting the electric signal is provided counts the passing frequency of the mark 6. Then, the result by counting is compared to the preset value by a comparator provided in the computer alarm 8 to compute the deviation, and when the deviation exceeds the specified value, an alarm provided in the computer alarm 8 is operated.