To detect an accurate paper feeding state of a sheet.
In a sheet detection device provided with a detection roller 32 displacing according to the thickness of the sheet S while abutting on the sheet and a detection means 33 detecting the thickness of the sheet by corresponding to the displacement of the detection roller 32, a flat part 24a along a belt 22 between two rollers 21 is provided to a conveyance means 20 conveying the sheet by the circulation of the belt 22, the endless belt 22 being wrapped around at least two rollers 21, and the detection roller 32 is also displaced so as to abut on the sheet conveyed on the flat part 24a. Therefore, the sheet can be stably conveyed on the flat part 24a, and the accurate paper feeding state of the sheet can be detected as the detection roller 32 displaces according to the sheet stably conveyed.