A fixing device includes a pressure roll having elasticity or flexibility which is a driving roller, a heat-resistant endless belt being in contact with the pressure roll to form a nip zone through which a recording medium passes, the heat-resistant endless belt rotating as driven by the pressure roll, a pressing supporting body disposed inside the heat-resistant endless belt and having a flat segment to press the heat-resistant endless belt for making the nip zone nearly flat, a heat source disposed inside the heat-resistant endless belt, the pressing supporting body being arranged between the heat source and the nip zone. Preferably, the heat-resistant endless belt is directly heated by radiant heat from the heat source and heated by thermal conduction through the pressing supporting body.