A fixing device including a heating roller; a pressure roller forming a nip section by making contact with the heating roller; a controller for controlling the surface temperature of the heating roller so that it becomes a predetermined set temperature; an amount of decrease detector for detecting an amount of decrease in the surface temperature of the pressure roller between before and after the passing of the paper sheet through the nip section; a determiner for specifying a required temperature required to perform the fixing for the paper sheet on the basis of the amount of decrease and for determining whether the set temperature is acceptable as the required temperature; and a temperature compensator for correcting the set temperature so that the set temperature is acceptable as the required temperature in the case that the determiner has determined that the set temperature is not acceptable as the required temperature.