PURPOSE: To prevent printing thinning from occurring at the time of a low temperature on a developer temperature controlling mechanism applied to an image forming device such as a printer and a copying machine, etc., of an electrophotographic system.

CONSTITUTION: The developer temperature controlling system 10 is constituted of a temperature detecting means 9 to detect the temperature of developer in a developing device, and a branching part 5 provided with a controlling valve 6. The branching part 5 is provided on the way of the exhaust pipe 12 of a blower 4 which absorbs the air of the periphery of a flash fixing unit 3 and exhausts it to the outside of the device, and also can exhaust the exhaust air of the blower 4 to the inside of the device. When the detected temperature of the temperature detecting means 9 is equal to or below a specified value, the controlling valve 6 is positioned on a second position so that the exhaust air is exhausted inside the device; and when the detected temperature is higher than the specified value, the controlling valve 6 is positioned on a first position, so that the exhaust air is exhausted to the outside of the device.