A fixing device includes: a fixing belt that heats a recording medium with a toner image transferred thereon to fix the toner image to the recording medium; a heater unit for heating the fixing belt; and a tension roller that suspends the fixing belt in a rotatable manner. This fixing device is constructed such that the heater unit is extended in the direction of the belt width that is perpendicular to the rotational direction of the fixing belt and includes a planar heat generator that generates heat by supplying electricity, a heat transfer member arranged in contact with both the planar heat generator and the fixing belt and a reinforcing member for fixing the heat transfer member while the heat transfer member is joined to the reinforcing member.