To widely and sufficiently ensure the efficiency of a variable turbo- charger from the low speed area of an engine to the high speed area thereof.

At an engine low speed area, a variable control valve 24 and a waste gate valve 26 of a variable control valve 24 of the variable turbo charger 20 are both kept in a full closed state. At an intermediate speed area, the variable control valve 24 is variably controlled between full closing and full opening with the waste gate valve 26 kept in a full closed state. And, at a high speed area, the waste gate valve is variably controlled between full closing and full opening with the variable control valve 24 kept in a full opening state.