A transporter is adapted to transport a sheet in a first direction. At least three detectors are disposed at a downstream side of the transporter in the first direction. Each of the detectors is operable to detect passing time of a leading end edge of the sheet. A processor is operable to calculate: a first angle of the sheet with respect to the first direction based on a first difference of the passing time detected by first two of the detectors and a first distance between the first two of the detectors; and a second angle of the sheet with respect to the first direction based on a second difference of the passing time detected by second two of the detectors and a second distance between the second two of the detectors, and operable to detect a jam in case that a value of an angular difference between the first angle and the second angle is larger than a prescribed value.