A toner image fixing device realizes a stabilized fixing process by smoothly controlling a surface temperature of its heating roller, preventing peeling-off of a layer of the roller core. The toner image fixing device comprises a heating roller consisting of a core covered with a layer of, e.g., a silicon rubber, a heat lamp mounted in the heating roller and a pressure roller for pressing a sheet of paper against the heating roller. The toner image fixing device is provided with a temperature sensor for sensing a surface temperature of the heat roller and for operating the heater lamp at a preset ON-OFF ratio (e.g., 1:3) to elevate the heating roller surface temperature from a first preset value to a second preset value and thereafter to maintain it at the second preset temperature.