When a thickness sensor detects a corner fold in a front end part of the paper sheet being carried, a light sensor checks whether a break exists or not in the front end part. When there is no break, it means the thickness sensor does not detect a corner fold but a foreign matter such as a tape mending a break. When there is a break, it means the thickness sensor detects a corner fold in the front end part. When there is no corner fold, the light sensor checks the front end part. When there is a break, it means there is a corner break but no corner fold in the front end part. When there is no break, it is a normal state. Regarding the back end part, a corner fold is detected in similarly to the foregoing.