An internal combustion engine that can ensure an adequate cooling performance by using a small number of component parts, and can be minimized in size. The cooling device includes a boiling water cooling device including a water jacket, coolant piping, steam piping and a radiator, an air cooling fan connected to one end of a crankshaft projecting from an outer surface of the engine main body, and a cover member provided on the engine main body so as to cover the air cooling fan, and define a cooling air passage extending to a radiator core.

A pressure relief valve is opened from a first time to a second time for the purpose of reducing the pressure in the gas phase in a gas-liquid separator. A first water pump (WP) is driven at a third time and a fourth time. The third time and fourth time correspond to timings at which a difference between a boiling temperature and an actual temperature becomes equal to or greater than a predetermined temperature. Since liquid-phase coolant in a catch tank can be fed to another water pump by driving the first WP, the actual temperature of the liquid-phase coolant immediately upstream of the other water pump can be lowered. It is thus possible to prevent intense boiling of the liquid-phase coolant immediately upstream of the other water pump.