A sheet conveying device includes an endless conveyor belt which has an electrode unit for retaining the sheet by using an electric force and which rotates while retaining the sheet so as to convey the sheet, a first electricity-supplying unit which applies a voltage to the electrode unit while it passes through a predetermined area, a second electricity-supplying unit which is disposed at a predetermined position which is upstream of the predetermined area and downstream of a position where the conveyor belt receives the sheet along a sheet-conveying path, and which applies another voltage to the electrode unit while it passes by the predetermined position, a detecting unit which detects a current or a voltage when the second electricity-supplying unit supplies electricity, and a determining unit which determines the state of the surface of the conveyor belt on the basis of the detected current or voltage.