A gas turbine engine comprising: an outlet guide vane downstream of a compressor; an outer casing supporting a radially outward part of the outlet guide vane; and an inner diffuser supporting a radially inward part of the outlet guide vane; wherein the outlet guide vane includes: a radially inward inner flange; a projecting part projecting radially inward from the inner flange; and an engagement part protruding to one side in an axial direction from a foreside of the projecting part; wherein the inner diffuser includes a smaller-diameter part having a smaller outer diameter than a part adjacent to the smaller-diameter part; wherein the inner diffuser is provided with an engagement groove extending to the one side in the axial direction from an outer peripheral surface of the smaller-diameter part or a region in the vicinity of the outer peripheral surface of the smaller-diameter part; wherein the engagement part is inserted into the engagement groove with a gap between the engagement part and the engagement groove, wherein a last-stage stator vane of the compressor is upstream of the outlet guide vane; wherein the outer casing supports a radially outward part of the stator vane; wherein the inner diffuser supports a radially inward part of the stator vane; wherein the stator vane includes: a radially inward stator vane inner flange; a projecting part projecting radially inward from a foreside of the stator vane inner flange; and a stator vane engagement part protruding to a downstream side from the projecting part; wherein the inner diffuser includes an engaged part at a foreside of the smaller-diameter part and protruding forward; and wherein the engaged part is between the stator vane engagement part and the stator vane inner flange. an outlet guide vane downstream of a compressor; an outer casing supporting a radially outward part of the outlet guide vane; and an inner diffuser supporting a radially inward part of the outlet guide vane; wherein the outlet guide vane includes: a radially inward inner flange; a projecting part projecting radially inward from the inner flange; and an engagement part protruding to one side in an axial direction from a foreside of the projecting part; wherein the inner diffuser includes a smaller-diameter part having a smaller outer diameter than a part adjacent to the smaller-diameter part; wherein the inner diffuser is provided with an engagement groove extending to the one side in the axial direction from an outer peripheral surface of the smaller-diameter part or a region in the vicinity of the outer peripheral surface of the smaller-diameter part; wherein the engagement part is inserted into the engagement groove with a gap between the engagement part and the engagement groove, wherein a last-stage stator vane of the compressor is upstream of the outlet guide vane; wherein the outer casing supports a radially outward part of the stator vane; wherein the inner diffuser supports a radially inward part of the stator vane; a radially inward inner flange; a projecting part projecting radially inward from the inner flange; and an engagement part protruding to one side in an axial direction from a foreside of the projecting part; wherein the inner diffuser includes a smaller-diameter part having a smaller outer diameter than a part adjacent to the smaller-diameter part; wherein the inner diffuser is provided with an engagement groove extending to the one side in the axial direction from an outer peripheral surface of the smaller-diameter part or a region in the vicinity of the outer peripheral surface of the smaller-diameter part; wherein the engagement part is inserted into the engagement groove with a gap between the engagement part and the engagement groove, wherein a last-stage stator vane of the compressor is upstream of the outlet guide vane; wherein the outer casing supports a radially outward part of the stator vane; wherein the inner diffuser supports a radially inward part of the stator vane; wherein the stator vane includes: a radially inward stator vane inner flange; a projecting part projecting radially inward from a foreside of the stator vane inner flange; and a stator vane engagement part protruding to a downstream side from the projecting part; a radially inward stator vane inner flange; a projecting part projecting radially inward from a foreside of the stator vane inner flange; and a stator vane engagement part protruding to a downstream side from the projecting part; wherein the inner diffuser includes an engaged part at a foreside of the smaller-diameter part and protruding forward; and wherein the engaged part is between the stator vane engagement part and the stator vane inner flange.