9 a of the intake port 9 if the positive pressure wave reaches the intake port throat 9 a when the intake valve 11 is open or in other words a synchronization between the pressure wave and the intake valve timing occurs it boosts cylinder air charge which is so called inertia supercharging effect the synchronization occurs if the sound speed times cycle time of the intake valve operation equals to an effective length for which the pressure wave transmits back and forth from the intake valve 11 the effective length can be calculated from an actual length of and respective sectional areas on a path of the transmission of the pressure wave in this embodiment the intake port 9 and the branch intake passages 32 and 33 are configured so that the synchronization occurs at a certain engine speed for example between 3000 and 4000 rpm if the positive pressure wave reaches the intake port throat 9 a while the intake valve 11 is closed the positive pressure wave reflects and turns its phase to be a negative pressure wave at the intake port throat 9 a the negative pressure wave bounces back from the surge tank 31 to.