PURPOSE: To quickly detect a paper jam and to prevent damage of elements by providing skew detecting means adapted to detect skew of paper according to the operation order of plural paper detecting means.

CONSTITUTION: When copy paper is transported, first a paper sensor PS1 is turned on and then a paper sensor PS2 is turned on. When the copy paper is further transported, the paper sensor PS1 is turned off and then the paper sensor PS2 is turned off. Thus, it is detected that the copy paper is normally transported. When a skew occurs, first the paper sensor PS1 is turned on, and then the paper sensor PS2 is turned on. After that, when the copy paper skews in the direction of the arrow, the paper sensor PS2 is turned off before the paper sensor PS1 is turned off. The skew of a copy paper is detected from the difference in the operation order to detect a paper jam.