An air intake system for an internal combustion engine uniformly distributes the fuel-air mixture in the cylinder, and concentrates the mixture in the vicinity of an ignition plug in the cylinder by selecting the direction of supply of sprayed fuel, thereby improving ignitability of the mixture at the time of lean-burn operation. The system produces tumble flow in the cylinder for accelerating combustion and provides lean-burn operation, thereby suppressing consumption of the fuel and reducing the amount of exhaust gas. The fuel is injected by a fuel injection valve and collides with an intake valve, before being atomized. The atomized fuel flows into the cylinder from the side of the intake valve closer to the fuel injection valve. On the other hand, high-velocity air flow from bypass passages enters the cylinder from the side of the intake valve closer to the ignition plug, so as to produce tumble flow in the cylinder.