Abstract: An intake system of an internal combustion engine is provided with a cross-sectional area changing means and a length changing means so that the cross-sectional area and/or the length of the intake system may be changed according to the change in the engine speed. Basically, at a high engine speed, the cross-sectional area is enlarged and the length is also enlarged to obtain an inertia supercharging effect at the high engine speed. By making the timing of changing the area different from that of changing the length, it is possible to maximize the inertia supercharging effect over a wide range of engine speeds. The cross-sectional area may be changed from SMALL to LARGE before the length is changed from LONG to SHORT, or after the length is changed from LONG to SHORT. The length may be changed stepwisely or continuously.