A fixing unit that heat-fixes the toner image onto the sheet, has: an information acquisition section that acquires information concerning a factor causing the input voltage which is supplied to a heater for heating the heating roller to vary; a temperature setting section that sets a target temperature of the heater based on the information acquired by the information acquisition section; and a temperature controller that controls the temperature of the heater such that the heating roller becomes the target temperature set by the temperature setting section.