To enhance the reliability in foreign matter detection and to prevent the occurrence of the rupture of paper sheets at the time of transportation.

This detector has transporting belts 12a, 12b which hold and transport the paper sheets 15 and pickup assemblies 16 to 18 for displacement detection consisting of pickups 16a to 18a for displacement detection which are disposed alongside these transporting belts 12a, 12b, are arrayed in juxtaposition at prescribed intervals in the transportation direction B and are lined up in parallel with the direction perpendicular to the transportation surface A. The respective pickup assemblies 16 to 18 for displacement detection have bearings 19 to advance and retreat the respective pickups 16a to 18a for displacement detection in the direction perpendicular to the transportation direction B on the transportation surface A and sensor 20 for displacement detection to be switched on and off by the advance and retreat of these bearings 19 and are installed to the positions where the pickups are offset from each other in the parallel lining up direction of the pickups 16a to 18a for displacement detection.