A fixing device includes a fixing member, a pressing member, a heating unit, multiple sub-heaters disposed in the heating unit, a temperature sensor to detect a temperature of the heating unit, and a heating control unit. The heating control unit controls the heating unit by controlling the multiple sub-heaters individually to heat the respective heating areas, such that a temperature at a portion of the fixing member corresponding to a blank area of the recording medium is lower than a temperature of a portion of the fixing member corresponding to an imaged area of the recording medium. The heating control unit further changes a size of a preliminary heating area to preliminarily heat the respective heating areas before the imaged area enters the fixing nip according to the temperature detected by the temperature sensor.