A construction machine comprising: an automotive lower traveling structure; an upper revolving structure swingably mounted on said lower traveling structure; and a working mechanism liftably mounted to said upper revolving structure, wherein said upper revolving structure is provided with a revolving frame serving as a support structure, a floor member provided on said revolving frame and having an operator's seat mounting section at the rear side for mounting an operator's seat and having a foot rest section at the front side on which an operator rests his or her feet, a cab box provided to cover the periphery and the upper side of said floor member for forming an operator's room on said floor member, an air conditioner unit provided at the lower surface side of said foot rest section to suck in internal air from an internal air inlet port (10A3) opened inside of said operator's room or suck in external air from an external air inlet port (10E3) opened outside of said operator's room for forming conditioned air which is supplied into said operator's room, and a suction duct is provided in said operator's seat mounting section of said floor member for circulating air toward a suction side of said air conditioner unit from said internal air inlet port (10A3) and said external air inlet port (10E3); characterized in that said suction duct comprises an internal air duct portion connected to said internal air inlet port (10A3), an external air duct portion connected to said external air inlet port (10E3), and a common duct portion for connecting said internal air duct portion, and said external air duct portion to the suction side of said air conditioner unit, and said suction duct is formed as a hollow column of a hollow closed section structure with said internal air duct portion and said common duct portion formed integrally and extended in the upper and lower directions, and said external air duct portion of said suction duct has a groove-shaped cross section, in which a face opposing said operator's seat mounting section of said floor member is opened, and is attached to said operator's seat mounting section to form a hollow closed section structure together with said operator's seat mounting section. an automotive lower traveling structure; an upper revolving structure swingably mounted on said lower traveling structure; and a working mechanism liftably mounted to said upper revolving structure, wherein said upper revolving structure is provided with a revolving frame serving as a support structure, a floor member provided on said revolving frame and having an operator's seat mounting section at the rear side for mounting an operator's seat and having a foot rest section at the front side on which an operator rests his or her feet, a cab box provided to cover the periphery and the upper side of said floor member for forming an operator's room on said floor member, an air conditioner unit provided at the lower surface side of said foot rest section to suck in internal air from an internal air inlet port (10A3) opened inside of said operator's room or suck in external air from an external air inlet port (10E3) opened outside of said operator's room for forming conditioned air which is supplied into said operator's room, and a suction duct is provided in said operator's seat mounting section of said floor member for circulating air toward a suction side of said air conditioner unit from said internal air inlet port (10A3) and said external air inlet port (10E3); characterized in that said suction duct comprises an internal air duct portion connected to said internal air inlet port (10A3), an external air duct portion connected to said external air inlet port (10E3), and a common duct portion for connecting said internal air duct portion, and said external air duct portion to the suction side of said air conditioner unit, and said suction duct is formed as a hollow column of a hollow closed section structure with said internal air duct portion and said common duct portion formed integrally and extended in the upper and lower directions, and said external air duct portion of said suction duct has a groove-shaped cross section, in which a face opposing said operator's seat mounting section of said floor member is opened, and is attached to said operator's seat mounting section to form a hollow closed section structure together with said operator's seat mounting section.