A fixing apparatus, comprising: a fixing member for a rotatable endless belt; a pressurizing member which is arranged to be abuttable against the fixing member on an outer peripheral side of the fixing member; a supporting member which is a C type pipe member and which has a concave portion with an opening thereof arranged to be a bottom face side, the C type pipe member being fixedly provided inside the fixing member and an outer peripheral face of the C type pipe member being in slidable contact with an inner peripheral face of the fixing member; a heating unit which heats the supporting member; a nip forming member which is contained in the concave portion of the supporting member that is on an inner peripheral side of the fixing member, and which abuts against the pressurizing member via the fixing member to form a nip portion; and a deform preventing unit which reinforces the concave portion to prevent deforming of the supporting member due to sliding of the fixing member, characterized in that the deform preventing unit includes an internal holding member which has a groove shape with a cross section which is U-shaped and has sleeve portions (71 S1, 71 S2) whose lengths differ, the internal holding member being installed from an outer peripheral side of the supporting member to inside the concave portion, wherein an outer face of a longer one (71 S1) of the sleeve portions is abutted against a side wall on a nip portion inlet side of the concave portion, wherein the deform preventing unit includes an external holding member which has a groove shape with a cross section which is U-shaped and has sleeve portions whose lengths differ, the external holding member being installed from an inner peripheral side of the supporting member to outside the concave portion, wherein an inner face of a longer one of the sleeve portions is abutted against a side wall on a nip portion outlet side of the concave portion, wherein that the internal holding member and the external holding member include a reverse mounting preventing unit which prevents a direction of mounting of the holding members (70, 71) in a direction of rotation of the fixing member from being a reverse direction. a fixing member for a rotatable endless belt; a pressurizing member which is arranged to be abuttable against the fixing member on an outer peripheral side of the fixing member; a supporting member which is a C type pipe member and which has a concave portion with an opening thereof arranged to be a bottom face side, the C type pipe member being fixedly provided inside the fixing member and an outer peripheral face of the C type pipe member being in slidable contact with an inner peripheral face of the fixing member; a heating unit which heats the supporting member; a nip forming member which is contained in the concave portion of the supporting member that is on an inner peripheral side of the fixing member, and which abuts against the pressurizing member via the fixing member to form a nip portion; and a deform preventing unit which reinforces the concave portion to prevent deforming of the supporting member due to sliding of the fixing member, characterized in that the deform preventing unit includes an internal holding member which has a groove shape with a cross section which is U-shaped and has sleeve portions (71 S1, 71 S2) whose lengths differ, the internal holding member being installed from an outer peripheral side of the supporting member to inside the concave portion, wherein an outer face of a longer one (71 S1) of the sleeve portions is abutted against a side wall on a nip portion inlet side of the concave portion, wherein the deform preventing unit includes an external holding member which has a groove shape with a cross section which is U-shaped and has sleeve portions whose lengths differ, the external holding member being installed from an inner peripheral side of the supporting member to outside the concave portion, wherein an inner face of a longer one of the sleeve portions is abutted against a side wall on a nip portion outlet side of the concave portion, wherein that the internal holding member and the external holding member include a reverse mounting preventing unit which prevents a direction of mounting of the holding members (70, 71) in a direction of rotation of the fixing member from being a reverse direction.