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.