When the conveying speed of a sheet by a first fixing device 9a disposed on the upstream side of the conveying path of the sheet is defined as Vu, and the conveying speed of the sheet by pairs of conveying rollers 31, 32 and 33 provided between the first fixing device 9a and a second fixing device 9b is defined as Vm, and the conveying speed of the sheet by the second fixing device 9b disposed on the downstream side is defined as Vl, Vu, Vm and Vl are set to the relation that Vu