PURPOSE: To easily detect the transfer failure by storing the thickness of a sheet detected by a detecting means on the paper feed section side, comparing it with the thickness detected by a detecting means on the paper discharge section side, and generating an alarm in case of inconsistency.

CONSTITUTION: Light permeability sensors S1, S2 are provided near the paper feed section 14 and near the paper discharge section 23 of a copying machine main body 1. The sensors S1, S2 are constituted with light emitters S1a, S2a and light receivers S1b, S2b, which are installed on both sides of the sheet to detect the thickness of the sheet via the light permeability. The thickness of the sheet detected by the sensor S1 is stored and compared with the thickness of the sheet detected by the sensor S2, and in case of inconsistency, the paper jam or duplicate feed is judged, thus an alarm is generated. Accordingly, detection of the transfer failure is facilitated.