A fixing device includes a fixing section, a sheet-width detector, a heating-width changing mechanism, a mixed feeding detector, and a controller. The fixing section includes at least a fixing member that fixes a toner image on a recording medium upon application of heat thereto. The sheet-width detector detects a width of the recording medium and outputs a sheet-width signal. The heat-width changing mechanism changes a heating width of the fixing member. The mixed feeding detector detects whether or not mixed feeding of a plurality of sheets of the recording medium having different widths is performed during a successive sheet feeding, and outputs a mixed feeding signal. The controller controls the heating-width changing mechanism to control the heating width based on the sheet-width signal and the mixed feeding signal.