To provide a high speed fixing device capable of performing fixation on the second side of a paper sheet without damaging a toner image formed on the first side, in the image forming device capable of performing the image formation on the paper sheet of both side.

In this fixing device provided with a heat roll 1 rotated in the direction of the arrow A and equipped with a heater 3 inside, the pressure roll 2 incorporating the heater 3 inside rotated in the direction of the arrow B opposite to the rotary direction A of the heat roll 1 while being held in contact with the heat roll 1, the device is disposing a cooling member 6 in an endless belt shape for cooling the pressure roll 2 surface, and the cooling member shifting device 10 capable of shifting the cooling device 6 to the first position P1 coming into contact with the pressure roll 2 surface, or shifting the cooling device 6 to the second position P2 parting from the pressure roll 2 surface, in accordance with an operating mode of the fixing device.