Blowby gas ventilation system for internal combustion engine

To provide a blowby gas ventilation system for an internal combustion engine capable of preventing oil from being diluted by efficiently and quickly exhausting a blowby gas together with a water content and the like from a crank chamber through forced ventilation of the crank chamber. The blowby gas ventilation system is intended for a four-stroke-cycle internal combustion engine. The system includes a fresh air introduction pipe (68), through which fresh air is sent from an outside of the internal combustion engine through a throttle portion (53) to a crank chamber (23a). The system also includes a blowby gas return passageway (70), through which the blowby gas is returned back to a downstream side of an air cleaner.