A paper-sheet-thickness detecting device includes a reference roller provided on a fixed rotation shaft; a detection roller provided to face and come into contact with the reference roller; a detection block in which the detection roller is provided at one end and the other end is rotatably fixed around a fulcrum shaft so that the detection block is turned and displaced according to a thickness of a paper sheet passing through between the reference and detection rollers; a holding block that holds the fulcrum shaft; a first pressing member fixed to the holding block to maintain contact between the detection and reference rollers by pressing the detection block, the first pressing member being displaced according to rotation and displacement of the detection block when the paper sheet passes through between the reference and detection rollers; and a displacement detector that detects a displacement amount of the first pressing member.