To provide a cutter for cutting a winding sheet to a prescribed form which detects a defective sheet that is mixed between piled sheets and cannot be detected only by inspecting corners of the piled sheets.

The cutter includes a proximity sensor for measuring a conveying timing of a sheet block and a sheet defective state detector which lowers a contact type displacement gauge in the direction of a sheet in accordance with information detected by the proximity sensor to measure the displacement of the thickness of the sheet block and detects the sheet block as an abnormal block when the displacement of the thickness of the sheet is extremely changed relative to the conveying direction of the sheet block.