A fixing device is equipped with a lower pressure roller, an upper pressure roller and a fixing belt which fix a toner image onto a sheet, by holding and conveying the sheet by a nip portion, and a control unit which controls rotation of the lower pressure roller. The control unit periodically changes a velocity of rotational drive of the lower pressure roller, at a frequency of which a period is shorter than a time needed for the sheet to pass through the nip portion, when the sheet passes through the nip portion.