A fixing device includes a rotatable endless belt, a heating source that heats the belt, a pad member having a sliding surface abutting on an inner surface of the belt, and a pressure roller that presses an outer surface of the belt toward the sliding surface and rotates to rotate the belt. The pressure roller works together with the belt to fix an unfixed toner image onto a medium that bears the unfixed toner image by allowing the medium to enter between the roller and the belt. Assuming that the sliding surface is divided into upstream and downstream regions in a moving direction of the belt, the upstream and downstream regions both have recessed curved surfaces in the moving direction. A first curved surface corresponding to the upstream region has an average curvature that is larger than that of a second curved surface corresponding to the downstream region.