Provided is a fixing device including: a fixing belt; a pressure roller which pressurizes toner on a sheet that passes through a pressure area; a pair of retention members each including a shaft-supporting portion which rotatably guides one of both end portions of the fixing belt in an axial direction, and a restriction surface which restricts a movement of the fixing belt in the axial direction; and a frame including a pair of side plates which rotatably support both end portions of the pressure roller in the axial direction as well as support the fixing belt via the pair of retention members, in which attachment positions of the pair of retention members in the pair of side plates are provided such that a gap becomes larger on a downstream side of a passing direction of the sheet with respect to the pressure area than on an upstream side.