A heating unit includes a rotating cylinder rotating about an axis parallel to a first direction. A heater having a width in a second direction is on the inner surface of the cylinder. A support member holds the heater fixed relative to the cylinder and includes an upstream member adjacent to the heater in the second direction and a downstream member adjacent to the heater in the second direction. The heating unit may include, in some examples, a stay supporting the support member, a guide member on the stay, and a temperature sensor to measure a temperature of an inner surface of the cylinder.