PURPOSE: To fix an envelope, etc., without causing wrinkle by forming a nip part between the fixing roller and the pressure roller so as to be linear in cross-section.

CONSTITUTION: In the fixing roller 1, the periphery of a hard core metal 4 is covered with an elastic body 5, such as Si rubber; a heating device 6, for example, a heater, is arranged in the core metal 4. In the pressure roller 2, a core metal 7 is covered with an elastic body 8, such as Si rubber, similarly to the fixing roller 1. The elastic body 5 of the fixing roller 2 is formed so that it is the same as the elastic body 8 of the pressure roller 2 in hardness and thickness; the nip part 9 between he fixing roller 1 and the pressure roller 2 is linear in the cross-section. Since the nip part 9 is linear, almost no difference in carrying speed between the papers which are sent while being superimposed results, and the cause of occurrence of wrinkle is eliminated.