An engine duct serving as a structural element of an aircraft engine, for taking in air and discharging a core jet and bypass jet, comprising: a cylindrical core cowl having an annular core passage for taking air therein and discharging the core jet; a cylindrical nacelle arranged outside the core cowl for surrounding the core cowl and having an annular bypass passage between an inner circumferential wall surface thereof and an outer circumferential wall surface of the core cowl for taking air therein and discharging the bypass jet; a plurality of struts arranged at intervals in a circumferential direction between the outer circumferential wall surface of the core cowl and the inner circumferential wall surface of the nacelle and supporting the nacelle with respect to the core cowl; a main pylon integrally connected from the core cowl to the nacelle, extending in parallel with an engine shaft direction of the aircraft engine, and projecting outward in a diametral direction from the nacelle characterised in that it comprises a sub-pylon being set between the outer circumferential wall surface of the core cowl and the inner circumferential wall surface of the nacelle in symmetry to the main pylon with respect to an engine shaft center and positioned between the struts that are adjacent to each other in the circumferential direction; and a plurality of fan outlet guide vanes arranged at intervals in the circumferential direction upstream from the struts between the outer circumferential wall surface of the core cowl and the inner circumferential wall surface of the nacelle and rectifying the air taken into the bypass passage into an axial flow, wherein: a projecting part is formed on the inner circumferential wall surface of the nacelle or on the outer circumferential wall surface of the core cowl so that the projecting part projects inward or outward in the diametral direction and extends from each circumferentially-oriented side face of at least one of the members including the sub-pylon, struts, and fan outlet guide vanes toward a downstream side; and the shape of the projecting part seen from the inner or outer side of the diametral direction is a streamline shape extending in the engine shaft direction and an apex of the projecting part is positioned on a rear edge of the at least one member (17, 13, 19). a cylindrical core cowl having an annular core passage for taking air therein and discharging the core jet; a cylindrical nacelle arranged outside the core cowl for surrounding the core cowl and having an annular bypass passage between an inner circumferential wall surface thereof and an outer circumferential wall surface of the core cowl for taking air therein and discharging the bypass jet; a plurality of struts arranged at intervals in a circumferential direction between the outer circumferential wall surface of the core cowl and the inner circumferential wall surface of the nacelle and supporting the nacelle with respect to the core cowl; a main pylon integrally connected from the core cowl to the nacelle, extending in parallel with an engine shaft direction of the aircraft engine, and projecting outward in a diametral direction from the nacelle characterised in that it comprises a sub-pylon being set between the outer circumferential wall surface of the core cowl and the inner circumferential wall surface of the nacelle in symmetry to the main pylon with respect to an engine shaft center and positioned between the struts that are adjacent to each other in the circumferential direction; and a plurality of fan outlet guide vanes arranged at intervals in the circumferential direction upstream from the struts between the outer circumferential wall surface of the core cowl and the inner circumferential wall surface of the nacelle and rectifying the air taken into the bypass passage into an axial flow, wherein: a projecting part is formed on the inner circumferential wall surface of the nacelle or on the outer circumferential wall surface of the core cowl so that the projecting part projects inward or outward in the diametral direction and extends from each circumferentially-oriented side face of at least one of the members including the sub-pylon, struts, and fan outlet guide vanes toward a downstream side; and the shape of the projecting part seen from the inner or outer side of the diametral direction is a streamline shape extending in the engine shaft direction and an apex of the projecting part is positioned on a rear edge of the at least one member (17, 13, 19). a projecting part is formed on the inner circumferential wall surface of the nacelle or on the outer circumferential wall surface of the core cowl so that the projecting part projects inward or outward in the diametral direction and extends from each circumferentially-oriented side face of at least one of the members including the sub-pylon, struts, and fan outlet guide vanes toward a downstream side; and the shape of the projecting part seen from the inner or outer side of the diametral direction is a streamline shape extending in the engine shaft direction and an apex of the projecting part is positioned on a rear edge of the at least one member (17, 13, 19).