If the temperature T1 of the middle part of a heat roller 2 is higher than the temperature T2 of the end parts of the heat roller 2 [T1.gtoreq.(T2-X)], the operating time B of side coils 5 and 6 is set longer than the operating time A of a center coil. If the temperature T1 of the middle part of a heat roller 2 is lower than the temperature T2 of the end parts of the heat roller 2 [T1<(T2-X)], the operating time A of the center coil is set longer than the operating time B of side coils 5 and 6.