At least, a temperature of the surface of a heating roller 13 in a region through which paper of small width passes, and a temperature of the surface of a heating roller 13 in a region through which paper of small width does not pass where paper of large width is carried are detected. Warming up operation is continued, and when a temperature of the surface of a heating roller 13 in the either region reaches a temperature capable of fixing, a ready display is lighted. Thereafter, the surface of the heating roller 13 not having reached a temperature capable of fixing is heated by forcibly lighting a heater lamp till the fixed conditions are satisfied, and at the time of the forcible lighting operation, the surface of the heating roller 13 in a region having reached a temperature capable of fixing is not heated.