To provide a device and a method for detecting the number of sheets capable of preventing the erroneous detection of double blanks, and preventing the slowdown of sheet feeding speed by a simple structure.

This device 10 for detecting the number of sheets to be fed, and measuring the weight of the sheet S fed to a press working line has: an idle stage 11 arranged between a sheet conveying line and a press machine carrying out an initial process in the press working line; a measuring means 15 arranged on the idle stage 11 and measuring the weight of the sheet S; and a controller 20 provided with a storage means to store the sheet S and a comparing means comparatively calculating the weight of the sheet S measured by the measuring means 15 and the unit weight of the sheet S stored in the storage means. When the measured value of the sheet S measured by the measuring means 15 exceeds a unit weight, it is determined that the number of the sheets S fed on the idle stage 11 exceeds one, and the supply of the sheet S is stopped.