In general, a fixing device according to an embodiment includes an endless belt, a pressure element, a heating member, an adjustment mechanism, and a controller. The pressure element conveys a sheet while interposing the sheet under pressure between the same and the endless belt. The heating member is provided on the inner side of the endless belt and has a heat generating element for heating the endless belt. The adjustment mechanism moves at least one of the heating member and the pressure element in such a direction as to bring the one closer to or away from the other, to adjust the nip width which is the length of an interposing and pressurizing region in a sheet conveyance direction, the interposing and pressurizing region being formed by the heating member and the pressure element to interpose the endless belt under pressure. The controller controls the adjustment mechanism so that A>B≥N is satisfied, where A is the nip width during a fixing process in which the sheet is heated to fix a color material onto the sheet, B is the nip width during temperature raising of the heating member to be conducted before the fixing process, and N is the length of the heat generating element in the sheet conveyance direction.