Provided is a compact boiling cooling system for an internal combustion engine that can operate in a stable manner. A lower end (4c) of a water jacket (4) of an engine (1) is connected to a lower tank (7c) of a radiator (7), and an upper end (4d) of a water jacket is connected to an upper part (7a) of the radiator. A substantially entire part of the radiator is located above the upper end of the coolant jacket so that the boiling cooling water is forwarded from the upper end of the water jacket to the upper part of the radiator, and the cooling water condensed in the radiator is forwarded from the lower end of the radiator to the lower end of the water jacket under the gravitational force without requiring a pump.