PURPOSE: To shorten an air intake duct so as to restrain fluctuations in air-fuel ratio caused by fluctuations in intake air by providing the air exit of an air cleaner near a mixer, and connecting the air exit of the air cleaner to the air inlet of the mixer by means of the air intake duct.

CONSTITUTION: A mixer 9 is connected to the mixed air inlet side end of an air intake manifold 5 which end is oriented in the longitudinal direction of the manifold 5. An air cleaner 20 is fixed to one end portion of a cylinder head 3 at roughly the same height as the mixer 9, which end portion is oriented in the longitudinal direction of the cylinder head 3. The air exit 20b of the air cleaner 20 is provided near the mixer 9 and is connected to the air inlet 9a of the mixer 9 by an air intake duct 24. The air intake duct 24 is thereby shortened and also the form of the duct 24 is simplified and so fluctuations in air-fuel radio caused by fluctuations in intake air within the air intake duct 24 are restrained so as to stabilize and enhance engine performance.