PURPOSE: To allow free set of a nip width and free alteration of fixation and, at the same time, to achieve the fixing of a toner image under heat without curling recording paper.

CONSTITUTION: The device is obtained by forming an elastically-deformable heat- resistant belt 20 made of for example metal into a cylinder. Inside the heat resistant belt 20 is a heater lamp 21 and a reflecting plate 22 which convergently heats, by means of the radiation heat, the vicinity of the part where the toner image and recording paper 8 holding the toner image on it are in contact. The device drives to bring the heat resistant belt 20, rotated and driven by a driving roller 24, into contact with the recording paper 8 carried on and along a guide plate 23 in the form of a flat plate. At this time, the rotating heat-resistant belt 20 and the guide plate 23, whose pressure can be moved, are allowed to move by rotating an eccentric cam 33 by means of an operation lever 35. By the radially elastic deformation of the heat resistant belt 20 when the heat resistant belt 20 and the guide plate 23 face and come into contact with each other, the recording paper 8 is pressed to the heat resistant belt 20, the area where the recording paper 8 is carried is made flat and, at the same time, the nip width in the carrying area can be freely changed. Thus, the toner image 28 is fixed under heat.