To prevent possibility of fog which may occur when a printing rate is high and to eliminate degradation in image quality due to the temperature of the inside of the device.

The toner supply device 16 is used for a color printer 1 of a system in which toner is supplied to each developing unit 13 from outside. The toner supply device 16 includes a toner mixture ratio sensor 21, a toner supply section 25, a printing rate detecting means 22, and an adjusting means 23. The toner mixture ratio sensor 21 detects a toner residual amount in the developing unit 13. Based on the toner residual amount, detected by the toner mixture ratio sensor 21, the toner supply section 25 supplies toner to the developing unit 13. The printing rate detecting means 22 detects a rate at which images are printed when image formation is carried out. When a printing operation in which a printing rate detected by the printing rate detecting means 22 is greater than a predetermined value is repeated, the adjusting means 23 increases an amount of toner supplied according to the printing rate.