A fixing device enabling the temperature of a heating element to be uniformly and stably maintained even if the heating element is eccentrically moved or vibrated. An opposed core (233) is so formed that a small diameter opposed core (2332) is fixedly inserted onto the center part of a core shaft (2331) and large diameter opposed cores (2333) are fixedly inserted onto both end parts of the core shaft (2331) so that the cross sectional area thereof at both ends in the lateral direction (both ends in the longitudinal direction) is larger than the cross sectional area thereof at the center part in the lateral direction. A fixing belt (210) is formed of a non-magnetic material and disposed between a core (232) and the opposed core (233). Thus, since a magnetic flux between the core (232) and the opposed core (233) is not almost varied and the fixing belt (210) allows the magnetic flux to pass therethrough and does not affect the flux, even if the fixing belt (210) is rotated and a distance between the core (232) and the fixing belt (210) is varied, the temperature of the fixing belt (210) can be uniformly and stably maintained.