A helically-shaped intake port comprising a helical portion formed around an intake valve, and a substantially straight inlet passage portion tangentially connected to the helical portion. A bypass passage is branched off from the inlet passage portion and connected to the helical portion. A rotary valve is arranged in the bypass passage and actuated by a vacuum operated diaphragm apparatus. The vacuum chamber of the diaphragm apparatus is selectively connected to the atmosphere or the suction pump driven by the diesel engine for opening the rotary valve when the level of load of the diesel engine is increased beyond a predetermined level.