A fixing apparatus including a heating member and a pressurizing member that are in contact with each other under pressure to define a nip through which a sheet carrying an unfixed toner image is passed and a temperature sensor for sensing the temperature of the heating member. The fixing apparatus further includes a temperature controller for proportionally controlling an electric power supplied to the heating member, based on the sensed temperature supplied from the temperature sensor, and for controlling the temperature of the heating member so that it is at a predetermined target temperature, and a heating condition changing device for changing a heating condition based on the target temperature and the sensed temperature when the temperature of the heating member is sensed. The electric power supplied to a halogen lamp functioning as a heat source of the heating roller is controlled in proportion to the temperature difference between the sensed temperature and the target temperature of the heating roller. The temperature of the pressurizing roller is estimated from the sensed temperature of the heating roller and the target temperature at a time when the sensing is performed. Then, heating conditions (such as the target temperature of the heating roller) which are suitable for the temperature of the pressurizing roller are reset.