PURPOSE: To efficiently control one engine by selectively controlling an intake system by means of changeover between an intake port below a cylinder and an overhead valve, and controlling the supercharging air systems of two turbo chargers by means of changeover between each connection in series and each connection in parallel.

CONSTITUTION: A intake port is opened when an engine is operated in a form of two-cycle, and intake valves 1a through 1d are opened/closed when the engine is operated in a form of four-cycle, and the exhaust system of the engine 1 is divided by two, so that the supercharging air systems of turbo chargers 3 and 4 disposed in each divided system are so constituted as to be freely connected mutually in series or in parallel. When the engine 1 is required to output power equal to or more than intermediate one, the engine shall be operated in a form of two-cycle in such a way that compressors 32 and 42 are so controlled as to be connected in series or in parallel. And when the engine is required to output power less than intermediate one, the engine shall be operated in a form of four-cycle in such a way that the compressors 32 and 42 are so controlled as to be connected in series or in parallel.