PURPOSE: To correctly monitor the travelling condition of a paper sheet so as to prevent a big trouble from occurring by providing a plural number of sensors to detect the overall length of a paper sheet in the travelling direction and the length of a space between adjoining paper sheets on the way of a conveyance route of the paper sheets.

CONSTITUTION: By comparing a signal concerning the overall length of a paper sheet 2 and the length of a space by a sensor 1 on the upstream side of the paper sheet 2 with a signal concerning the overall length of the paper sheet 2 and the length of a space by a sensor 11 on the downstream side, the travelling condition of the paper sheet 2, that is, whether it is travelling at a constant speed or not and whether there is any slant travel is detected. Accordingly, clogging of the paper sheet 2, irregularity of travelling speed, falling-off from a conveying conveyor, overlapping, jamming, etc. can be detected by a plural number of the sensors 1 and 11 provided o the conveyance route of the paper sheet and consequently, before such a big trouble as generation of a damage to the paper sheet, such proper measures as to stop conveying it etc. can be employed.