A fixing device including: a flexible cylindrical member; a heater that is disposed inside the cylindrical member; a nip plate configured to be in contact with an inner peripheral surface of the cylindrical member and is configured to be heated by the heater; and a roller configured to pinch the cylindrical member between the roller and the nip plate, wherein, at least during fixing, the nip plate has a curved shape where a central portion thereof in an axial direction of the cylindrical member is more spaced from the roller than both end portions thereof in the axial direction, and wherein the roller is a concave roller where a diameter of a central portion thereof in the axial direction is smaller than diameters of both end portions thereof respectively.