A fixing device includes a fixing roller configured to be heated by a heat source, and a pressure roller configured to be in pressure-contact with the fixing roller. The fixing roller includes a cylindrical tubular core having an inner circumferential surface and one or more ribs protruded from the inner circumferential surface and extending spirally along the inner circumferential surface. The total number of times that the one or more spiral ribs cross through a region of contact between the fixing roller and the pressure roller is more than one, regardless of a rotation angle of the fixing roller.