PURPOSE: To enable to obtain a high torque over a wide range of operation of an engine from its low-speed region to a high-speed region, by employing such an arrangement that one of two intake ports opened in a combustion chamber is connected to an intake pipe having a length o suit a high-speed operation of the engine at which the maximum output is obtained and the other intake port is communicated to another intake pipe having a length to suit a lower speed operation of the engine.

CONSTITUTION: Two intake ports 6 are opened in a combustion chamber 5 and these intake ports 6 are opened and closed by respective intake valve 8. For each cylinder of an engine, the intake ports 6 are communicated respectively to a first intake branch pipe 11 and a second intake branch pipe 12, and the other ends of these branch pipes 11, 12 are both opened in an intake air distributing box 14. The inner space of the distributing box 14 is divided into an upper chamber 14a and a lower chamber 14b by a partition wall 15, and the first branch pipe 11 is communicated to the upper chamber 14a while the second branch pipe 12 is communicated to the lower chamber 14b. Here, since arrangement is such that the first branch pipe 11 is slightly longer than the second branch pipe 12, the former pipe 11 is suited for relatively low speed operation of the engine while tha latter is suited for relatively high speed operation of the engine.