To perform both skew angle detection and jamming detection with simple constitution.

Two sensors 33 and 34 arranged in a conveyance path for conveying rectangular paper P are positioned differently both in a conveying direction and a main scanning direction. The sensor 33 detects that one side edge of the paper P exists at a first point. The sensor 34 detects that one side edge of the paper P exists at a second point. A skew amount of the paper P is calculated based on a time difference between the time when the sensor 33 detects the one side edge of the paper P and the time when the sensor 34 detects the one side edge of the paper P, and also occurrence of jamming of the paper P is detected based on whether or not the sensor 34 detects the one side edge of the paper P within the predetermined time from the time when the sensor 33 detects the one side edge of the paper P.