To reduce the number of components and the cost and to improve the output performance of an engine without reducing the arrangement freedom degree of a case by integrating a duct seal for connecting an intake introduction duct with a connecting tube continuing to a carburetor and setting it to a seal- tube combined body of a single component.

Intake air sucked into an intake air introduction duct D is introduced into an air cleaner case 1 through an internal space of the duct D and a duct seal S (air inlet 2i) and filtered by passing through an air filter 5. The clean intake air after filtration is fed to a connecting tube C from an air outlet 2o and introduced into a carburetor Ca. In this case, the duct seal S and the connecting tube D are integrally molded by an elastic material to be formed into a seal-tube combined body U. This constitution can reduce the number of the components and the cost, enlarge the diameters of the air inlet 2i and the air outlet 2o by approaching them to each other, and improve the output purity of the engine without enlarging the air cleaner case 1.