Fixing device and image forming apparatus

A fixing device comprising:

• an endless belt;
• a heat supply part that is disposed inside the endless belt, contacts an inner circumferential surface of the endless belt, and supplies heat to the endless belt;
• and
• a pressure member that contacts an outer circumferential surface of the endless belt to form a nip portion, and conveys a recording medium while nipping the recording medium together with the endless belt,
• the heat supply part comprising:
• a heater having a plurality of heating sections arranged in a widthwise direction of the endless belt, and
• a thermal diffusion member that extends in the widthwise direction between the heater and the endless belt, and is disposed so that one surface of the thermal diffusion member contacts the inner circumferential surface of the endless belt, and the other surface of the thermal diffusion member contacts the plurality of heating sections,
• wherein a plate thickness t (mm) between the one surface and the other surface of the thermal diffusion member, and a thermal diffusivity D (mm2/s) of the thermal diffusion member satisfy the relationship:
• t×D≥6.7.