Abstract: Two thickness detectors (5) each for outputting an electric signal representing the thickness of a paper sheet (B) passing thereby are arranged side by side in the width direction of a paper sheet conveyance path (3). Parameters such as paper sheet thickness, length, the existence of skew and, if skew exists, the skew angle (.theta.), are calculated based on the output signals from the thickness detectors. This facilitates detection of two or more overlapping bank notes among a wide variety of bank notes having different thicknesses or in case of a bank note having a significantly non-uniform thickness. The correct thickness of a paper sheet is also obtained at all times by using the skew angle and the reference length (W) of the paper sheet. This not only facilitates detection of two overlapping bank notes among a wide variety of bank notes having different thicknesses or in case of a bank note having a significantly non-uniform thickness, but also enables detection to be made very accurately.