To provide an image forming device, particularly suitable for a fixing device including a heating roller with a small heat capacity, which can raise a temperature of the heating roller at the time of startup to a target temperature without causing an overshoot, so that a fixing process after the startup can be performed quickly at an appropriate temperature.

A fixing device is provided with a heating roller, a pressure roller, a heat source, a temperature detecting element which detects the surface temperature of each roller, and a control unit which controls a temperature of the heat source. The control unit comprises a heating time calculation unit which calculates an energization time to the heat source at the time of startup. The heating time calculation unit, using the temperature of each roller at the time of startup as a parameter, calculates the energization time to the heat source, the time required for each roller to reach a target temperature. The calculation of the energization time is made in consideration of heat quantity of each roller determined based on a preset target temperature, a heating value generated by the heat source, and an actual measured value. The control unit keeps the heat source energized in accordance with a determined energization time.