A fixing device is provided which includes a fixing belt rotatably arranged, a fixing roller disposed to face the fixing belt and forming a fixing nip together with an outer surface of the fixing belt a guide unit configured to guide rotation of the fixing belt in the fixing belt; and a pair of support units configured to limit movement of the fixing belt in a width direction at both ends of the guide unit. The fixing device may have a structure of guiding rotation of the fixing belt and a structure of limiting movement thereof in width direction, which are separated from each other.