Abstract: A displacement detection device which detects the displacement of a paper sheet in transit in a paper sheet sorter that picks up and transports the paper sheet such as the bank-notes. The displacement detection device includes photo sensors which detect the leading edge of the paper sheet that is being transported, and a photo position detector which detects the distance from the conveyer belt to the side edge of said paper sheet. Each of said photo sensors comprises of a light emitter and a light receiver which are arranged on opposite sides of the paper sheet. The photo position detector is placed on the downstream side in the conveyance direction of the photo sensors, extending perpendicularly to the conveyance line, and comprises a light projector and a light receiver with linear form and equal length arranged symmetrically with respect to the paper sheet.