An exhaust duct has a riser duct section which is connected to an exhaust port having such an inlet port at a lower portion of the riser duct section as is connected to an exhaust port for the combustion gas. The riser duct section extends upward along an external surface of the combustion box. A horizontal direction perpendicular to the front-to-back direction is defined as a lateral direction, then the riser duct section is formed into a flat shape having a smaller dimension in the front-to-back direction than the dimension in the lateral direction. Fluctuations in width between front-side and back-side plate parts of the riser duct section are devised to be restrained, and increase in size can be avoided. The riser duct section has a rail inside the riser duct section, the rail being elongated in the vertical direction and connecting together the front-side and the back-side plate parts.