PURPOSE: To provide an image forming device having both characteristics of a thermal fixing device satisfactory in fixed image quality and fixing performance but requiring much time for warming up and large in power consumption and a pressure fixing device good in instantaneous printability but bad in the fixing performance of the image quality.

CONSTITUTION: The pressure fixing roller 3 and the pressing roller 4 are brought into press-contact with the thermal fixing roller 1, and a fixing entrance flapper 17 is provided. When the fixing entrance flapper 17 is on a position expressed by a continuous line in the figure, a heater 2 is not energized, and a transferring material fed from the right part of the figure while supporting an unfixed image is pressured and fixed. When the fixing entrance flapper 17 is on a position expressed by an alternate long and two short dashes line in the figure, the heater 2 is energized, so that the transferring material is thermally fixed. Either fixing method can be selectively used by an user, or the fixing entrance flapper 17 can be automatically switched between pressure fixing in the case the temperature of the roller 1 is equal to or below a fixing temperature and thermal fixing in the case where the temperature of the roller 1 is equal to or above the fixing temperature.