A heat roller fixing device which can substantially make uniform the amount of deflection of a backup roller over the axial length thereof to improve a paper feed force at an axially central portion of the backup roller. The heat roller fixing device includes a heat roller rotatably mounted to a frame, and a backup roller pressed against a heat roller by coil springs to follow rotation of the heat roller. The backup roller includes a hollow cylinder, a solid metal shaft inserted in the hollow cylinder and having a pair of large-diameter portions spaced a given distance from each other. One of the large-diameter portions of the solid metal shaft is fixed to the hollow cylinder such as by a pin therethrough, and the solid metal shaft is biased toward the heat roller by the coil springs.