There are provided first and second water supply passages to supply cooling water from an engine to first and second center housings of first and second turbochargers, and first and second return passages to return the cooling water from the first and second turbochargers to the engine. A cooling-water connection portion of the first water supply is located above the level of a cooling-water connection portion of the second water supply passage. A vapor releasing passage is provided between the second turbocharger and an upper tank provided on the outside of an engine body at a position located above the connection portion of the second return passage of the second turbocharger. Accordingly, the function of vapor releasing from the first and second turbochargers can be improved, thereby increasing the layout flexibility around the engine.