A fixing device is provided, which has a fixing roller and a sheet. A recording paper where a toner image has been transcribed thereon is passed through a nip portion between the fixing roller and the sheet. By pressing and heating, the toner image is fixed on the recording paper. A heating line is wound on a surface opposite to a sheet's surface that is in contact with the fixing roller. By applying a power source to the heating line, the sheet is heated. When the recording paper enters the nip, the cause of the temperature suddenly lowering can be prevented.