PURPOSE: To detect jam state in reasonable manners without requiring the adjusting work for a paper having a variety of size by installing a timing means which sets the allowable detection time for a paper part and an interval part on the basis of the length set by a paper length setting means.

CONSTITUTION: The existence of a paper 5 and the interval between papers are detected by a sensor 7, and the jam state of the paper is detected. If the paper size is changed in this case, a timing means 10 sets the allowable detection time for the paper and the interval by the sensor 7, receiving the signal of a paper length setting means 8. Therefore, jam detection can be carried out in reasonable manners without requiring the adjusting work for the paper having a variety of size.