A water jacket in the turbo-charger is positioned at the same level or higher than a water jacket in an engine cylinder head. The turbo-charger cooling water circulation passageway includes a water volume or tank positioned at a level higher than the turbo-charger cooling portion, and the volume or tank is connected to a cooling water reservoir tank via a pressure relief valve
which is opened when pressure in the volume or tank exceeds a predetermined value to supply cooling water to the volume or tank.