PURPOSE: To improve the performance of an internal combustion engine in an extensive load range by installing a first valve in a main passage lying between both interconnecting parts of a sub passage and a first throttle body, and setting up a second valve for opening or closing an interval between both first and second throttle bodies.

CONSTITUTION: A main passage 23 is interconnected to a cylinder head 21 of an internal combustion engine. The other end of this main passage 22 is interconnected to a first chamber 24 as a first throttle body. This first chamber 24 is linked up with an intake upstream part via a first throttle valve 25. In addition this first chamber 24 is linked up with a second chamber 27 as a second throttle body. This second chamber 27 is linked up with the intake upstream part via a second throttle valve 28. In this case, this second chamber 27 is interconnected to the main passage 22 through a sub passage 29. In succession, a first valve 30 is installed in the main passage 22 lying between both interconnecting parts of this sub passage 29 and the first chamber 24. Thus, the extent of performance in the internal combustion engine is improvable in an extensive load range.