A fixing device includes a cylindrical film, and a heater including a substrate, a first heat generating resistor, and a second heat generating resistor. A controller controls supply of electric power to the heater using one of PID control and PD control on the basis of a deviation between a target temperature and a detected temperature detected by a temperature detecting member, such that the temperature of the heater is maintained at the target temperature. A proportional gain of the PID control or the PD control when the first heat generating resistor is supplied with the electric power and the second heat generating resistor is not supplied with the electric power is less than a proportional gain of the PID control or the PD control when the first heat generating resistor is not supplied with the electric power and the second heat generating resistor is supplied with the electric power.