An intake system for an internal combustion engine, having a structure in which blow-by oil and/or fuel entering into an air cleaner case can be discharged to the outside of the air cleaner case. The intake system for an internal combustion engine is provided in which air is sucked from a clean air chamber in the air cleaner case into a throttle body provided with a throttle valve through a funnel while defining an airflow passage. Concurrently, fuel is sprayed from an injector, thereby feeding the fuel-air mixture into an intake port. A bypass passage different from the airflow passage is provided between the clean air chamber in the air cleaner case and the throttle body. The bypass passage is so structured that a passage inlet thereof on the clean air chamber side opens at an inner wall surface of the air cleaner case.