In order to (i) minimize the amount of liquid coolant reaching a condenser or radiator of an internal combustion engine or the like in which the coolant is permitted to boil and the vapor used as a vehicle for removing heat, and to (ii) simultaneously maintain the level of liquid coolant in the jacket above the combustion chambers, exhaust port and valves of the engine; a drain passage is provided between (a) the collection chamber of a manifold into which the vapor is discharged from the coolant jacket via manifold runners, and (b) the coolant jacket. This drain passage passage by-passes the runners and vapor discharge ports of the coolant jacket, connects with the coolant jacket at a level lower than the engine structure which is subject to high heat flux and houses a coolant level control sensor.