A fixing device comprising: an endless fixing belt that is rotatable at a speed substantially equal to a convey speed of a recording medium; a heat portion that heats the fixing belt; a pressure roller that is disposed to oppose an outer circumferential surface of the fixing belt and gives a rotation-drive force to the fixing belt; a push member that contacts an inside of the fixing belt to push the fixing belt against the pressure roller at a predetermined pressure; and a push pad that is disposed on a surface of the push member which opposes the fixing belt, includes a slide surface on which an inner circumferential surface of the fixing belt slides; wherein the recording medium is inserted into a fixing nip portion formed by the fixing belt and the pressure roller, whereby an unfixed toner image carried on the recording medium is fixed; the slide surface includes a flat portion substantially parallel to a convey direction of the recording medium and an arc-shaped portion that bends toward the pressure roller in a more downstream side than the flat portion in the convey direction with respect to the recording medium, wherein the fixing nip portion is formed such that the flat portion and the arc-shaped portion become continuous from an upstream side in the convey direction of the recording medium; the arc-shaped portion includes a radius R2 of curvature larger than a radius R1 of curvature of the pressure roller, and the pressure roller butts the arc-shaped portion and a part of the flat portion via the fixing belt at a predetermined pressure to form the fixing nip portion;, the arc-shaped portion has a center 02 of curvature that is situated in a more upstream side than a straight line which passes through a center O1 of curvature of the pressure roller and is perpendicular to the convey direction of the recording medium; said fixing nip portion is formed such that a boundary between the flat portion and the arc-shaped portion lies on the straight line which passes through the center O1 of curvature of the pressure roller and which is perpendicular to the convey direction of the recording medium; and an angle between the convey direction of the recording medium in an upstream side of the fixing nip portion and the convey direction of the recording medium in a downstream side of the fixing nip portion is 8° to 17°. an endless fixing belt that is rotatable at a speed substantially equal to a convey speed of a recording medium; a heat portion that heats the fixing belt; a pressure roller that is disposed to oppose an outer circumferential surface of the fixing belt and gives a rotation-drive force to the fixing belt; a push member that contacts an inside of the fixing belt to push the fixing belt against the pressure roller at a predetermined pressure; and a push pad that is disposed on a surface of the push member which opposes the fixing belt, includes a slide surface on which an inner circumferential surface of the fixing belt slides; wherein the recording medium is inserted into a fixing nip portion formed by the fixing belt and the pressure roller, whereby an unfixed toner image carried on the recording medium is fixed; the slide surface includes a flat portion substantially parallel to a convey direction of the recording medium and an arc-shaped portion that bends toward the pressure roller in a more downstream side than the flat portion in the convey direction with respect to the recording medium, wherein the fixing nip portion is formed such that the flat portion and the arc-shaped portion become continuous from an upstream side in the convey direction of the recording medium; the arc-shaped portion includes a radius R2 of curvature larger than a radius R1 of curvature of the pressure roller, and the pressure roller butts the arc-shaped portion and a part of the flat portion via the fixing belt at a predetermined pressure to form the fixing nip portion;, the arc-shaped portion has a center 02 of curvature that is situated in a more upstream side than a straight line which passes through a center O1 of curvature of the pressure roller and is perpendicular to the convey direction of the recording medium; said fixing nip portion is formed such that a boundary between the flat portion and the arc-shaped portion lies on the straight line which passes through the center O1 of curvature of the pressure roller and which is perpendicular to the convey direction of the recording medium; and an angle between the convey direction of the recording medium in an upstream side of the fixing nip portion and the convey direction of the recording medium in a downstream side of the fixing nip portion is 8° to 17°.