PURPOSE: To suppress the erroneous detection of jam when it is judged that the paper detecting region for a paper on which the light supplied from a luminous element is projected has the high depth which is sufficient for the erroneous detection by a paper detecting device.

CONSTITUTION: In the step S1, the depth in a nonimage region of a paper sheet is read out. It is judged if the read-out depth processed in the step S2 is larger than a prescribed depth or not. If the read-out depth is larger than the prescribed depth, it is judged that the depth is large because of the state where the paper sheet is set at a prescribed position of a cooying film and a large quantity of black toner is left on the paper sheet or the paper sheet is set nearly in this state, and in the step S3, a signal for prohibitung the jam detecting work is outputted. Accordingly, it is judged that the paper sheet is set on a copying film, and the jam detecting work is prohibited.