A variable intake air device forms a long port and a short port in each of branches of an intake air manifold. The variable intake air device switches between the long port and the short port. A gas introduction device draws gas into each of the branches. When the engine is in a high load region, the variable intake air device uses the long port, when an engine rotation speed is less than a predetermined rotation speed, and uses the short port, when the engine rotation speed is greater than or equal to the predetermined rotation speed. When the engine is in a low load region, the variable intake air device switches between the long port and the short port according to the engine rotation speed.