A fixing device includes a fixing belt for fixing an unfixed toner image onto a recording medium, a planar heating member for heating the fixing belt, and a pressure roller. The heating member includes a ceramic heat generating element having a PTC characteristic, and a high-thermal-conductive heat diffusion member. The fixing belt is formed in an endless shape and is supported around, at least, the high-thermal-conductive heat diffusion member, thereby to be heated. The ceramic heat generating element comes into contact with the fixing belt over the full width thereof with the high-thermal-conductive heat diffusion member interposed therebetween. The high-thermal-conductive heat diffusion member comes into contact with the fixing belt over the full width thereof and diffuses heat generated by the heat generating element, in the traveling direction of the fixing belt.