An outboard engine unit having incorporated therein an exhaust passage system for discharging exhaust gas from an engine into water, the exhaust passage system comprising: an engine-side exhaust pipe provided adjacent to the engine; a downstream exhaust pipe provided downstream of the engine-side exhaust pipe for directing the exhaust gas downstream from the engine-side exhaust pipe, a lower end portion of the engine-side exhaust pipe and an upper end portion of the downstream exhaust pipe being positioned in parallel to each other in such a manner that respective vertical central axes (27,28) of the lower end portion and the upper end portion are offset from each other in a substantially horizontal direction of the unit, characterised by: the lower end portion of the engine-side exhaust pipe and the upper end portion of the downstream exhaust pipe overlapping each other in the horizontal direction so as to provide fluid communication therebetween via a through-hole formed in overlapping portions of the end portions (31,32), the overlapping portions of the end portions (31,32) having a vertical length; and a vertical partition wall provided in an upper end region of the overlapping portions of the end portions of the engine-side exhaust pipe and the downstream exhaust pipe, being arranged to hamper back-flows of fluid into the engine-side exhaust pipe, the vertical partition wall having a length smaller than the vertical length of the overlapping portions of the end portions (31,32). an engine-side exhaust pipe provided adjacent to the engine; a downstream exhaust pipe provided downstream of the engine-side exhaust pipe for directing the exhaust gas downstream from the engine-side exhaust pipe, a lower end portion of the engine-side exhaust pipe and an upper end portion of the downstream exhaust pipe being positioned in parallel to each other in such a manner that respective vertical central axes (27,28) of the lower end portion and the upper end portion are offset from each other in a substantially horizontal direction of the unit, characterised by: the lower end portion of the engine-side exhaust pipe and the upper end portion of the downstream exhaust pipe overlapping each other in the horizontal direction so as to provide fluid communication therebetween via a through-hole formed in overlapping portions of the end portions (31,32), the overlapping portions of the end portions (31,32) having a vertical length; and a vertical partition wall provided in an upper end region of the overlapping portions of the end portions of the engine-side exhaust pipe and the downstream exhaust pipe, being arranged to hamper back-flows of fluid into the engine-side exhaust pipe, the vertical partition wall having a length smaller than the vertical length of the overlapping portions of the end portions (31,32). the lower end portion of the engine-side exhaust pipe and the upper end portion of the downstream exhaust pipe overlapping each other in the horizontal direction so as to provide fluid communication therebetween via a through-hole formed in overlapping portions of the end portions (31,32), the overlapping portions of the end portions (31,32) having a vertical length; and a vertical partition wall provided in an upper end region of the overlapping portions of the end portions of the engine-side exhaust pipe and the downstream exhaust pipe, being arranged to hamper back-flows of fluid into the engine-side exhaust pipe, the vertical partition wall having a length smaller than the vertical length of the overlapping portions of the end portions (31,32).