A fixing device, comprising: a frame defining a space in which the fixing device is accommodated; a tubular belt holder extending in an axial direction thereof; a rotatable, flexible fuser belt looped into a generally cylindrical configuration around the belt holder to rotate in a circumferential direction of the belt holder; a heater disposed adjacent to the belt holder to heat the belt holder to in turn heat the fuser belt through conduction; a fuser pad accommodated in the belt holder inside the loop of the fuser belt, and extending in the axial direction of the belt holder; a pressure member disposed opposite the belt holder with the fuser belt interposed between the fuser pad and the pressure member, the pressure member pressing in a load direction against the fuser pad through the fuser belt to form a fixing nip therebetween, through which a recording medium travels in a conveyance direction under heat and pressure; and a mounting attachment provided to a longitudinal end of the tubular belt holder to allow retention and detachable attachment of the belt holder end to the frame, characterized in that the attachment includes: a flange adapted to be affixed to the frame; and a tubular insert extending from the flange to be inserted into the longitudinal end of the belt holder, the tubular insert having a free axial end thereof at least partially protruding and tapered to form a tapered guide surface, along which an inner circumferential surface of the belt holder is slidably guided onto an outer circumferential surface of the tubular insert being inserted into the longitudinal end of the belt holder, the tapered guide surface consists of a first, central surface (28d1) and a pair of second, adjoining surfaces (28d2) on opposed sides of the first surface (28d1), wherein the first surface (28d1) tapers in the axial direction and has a substantially uniform depth in the circumferential direction, whereas the second surfaces (28d2) taper not only in the axial direction but also in the circumferential direction, so that the depth of the tapered guide surface is greatest at the first surface (28d1) and becomes narrower toward the distal end of the second surfaces (28d2). a frame defining a space in which the fixing device is accommodated; a tubular belt holder extending in an axial direction thereof; a rotatable, flexible fuser belt looped into a generally cylindrical configuration around the belt holder to rotate in a circumferential direction of the belt holder; a heater disposed adjacent to the belt holder to heat the belt holder to in turn heat the fuser belt through conduction; a fuser pad accommodated in the belt holder inside the loop of the fuser belt, and extending in the axial direction of the belt holder; a pressure member disposed opposite the belt holder with the fuser belt interposed between the fuser pad and the pressure member, the pressure member pressing in a load direction against the fuser pad through the fuser belt to form a fixing nip therebetween, through which a recording medium travels in a conveyance direction under heat and pressure; and a mounting attachment provided to a longitudinal end of the tubular belt holder to allow retention and detachable attachment of the belt holder end to the frame, characterized in that the attachment includes: a flange adapted to be affixed to the frame; and a tubular insert extending from the flange to be inserted into the longitudinal end of the belt holder, the tubular insert having a free axial end thereof at least partially protruding and tapered to form a tapered guide surface, along which an inner circumferential surface of the belt holder is slidably guided onto an outer circumferential surface of the tubular insert being inserted into the longitudinal end of the belt holder, the tapered guide surface consists of a first, central surface (28d1) and a pair of second, adjoining surfaces (28d2) on opposed sides of the first surface (28d1), wherein the first surface (28d1) tapers in the axial direction and has a substantially uniform depth in the circumferential direction, whereas the second surfaces (28d2) taper not only in the axial direction but also in the circumferential direction, so that the depth of the tapered guide surface is greatest at the first surface (28d1) and becomes narrower toward the distal end of the second surfaces (28d2). a flange adapted to be affixed to the frame; and a tubular insert extending from the flange to be inserted into the longitudinal end of the belt holder, the tubular insert having a free axial end thereof at least partially protruding and tapered to form a tapered guide surface, along which an inner circumferential surface of the belt holder is slidably guided onto an outer circumferential surface of the tubular insert being inserted into the longitudinal end of the belt holder, the tapered guide surface consists of a first, central surface (28d1) and a pair of second, adjoining surfaces (28d2) on opposed sides of the first surface (28d1), wherein the first surface (28d1) tapers in the axial direction and has a substantially uniform depth in the circumferential direction, whereas the second surfaces (28d2) taper not only in the axial direction but also in the circumferential direction, so that the depth of the tapered guide surface is greatest at the first surface (28d1) and becomes narrower toward the distal end of the second surfaces (28d2).