A fixing device includes an endless belt. A heat supply part is disposed inside the belt, contacts an inner circumferential surface thereof, and supplies heat thereto. A pressure member contacts an outer circumferential surface of the belt to form a nip portion, and conveys a recording medium while nipping the recording medium together with the belt. The heat supply part includes a heater having a plurality of heating sections arranged in a widthwise direction of the belt, and a thermal diffusion member extending in the widthwise direction between the heater and the belt. One surface of the thermal diffusion member contacts the inner circumferential surface of the belt, and the other surface contacts the heating sections. A plate thickness t (mm) between the surfaces of the thermal diffusion member, and a thermal diffusivity D (mm2/s) of the thermal diffusion member satisfy the relationship: t×D≥6.7.