A fixing device includes a fixing belt, a first heater, a second heater, a supporting roller, and a counter roller. During activation of the fixing device, a maximum power is supplied to the first heater, and only the first heater heats the fixing belt. When a temperature on the fixing belt reaches a first reference temperature, the fixing belt is driven rotationally, the power supplied to the first heater is dropped, and power is supplied also to the second heater. When the temperature on the fixing belt reaches a target temperature and the temperature on the counter roller reaches a second reference temperature, fixing operation is allowed to start.