PURPOSE: To obtain large stable brake power by constituting an engine of a power turbine, exhaust brake and opening and closing valve provided respectively interposing in the upstream of connection parts of an exhaust bypass passage making a detour of this power turbine and a bypass passage relieving air just in the upstream of the power turbine when it is in an overspeed.

CONSTITUTION: When a power turbine 7 is inverted from normal rotation to reverse rotation by an inverting mechanism 17, a brake valve 13 and an opening and closing valve 14 in an exhaust bypass passage 8 are respectively closed. Then because the turbine 7 stirs exhaust gas in an exhaust passage 5 performing work of mixing the gas by stirring it, that is, negative work, here brake power obtained by an increase of exhaust pressure of an engine and brake power obtained by the work of mixing by stirring are generated. Since a bypass passage 60 releases air just in the upstream of the turbine 7, when it is in an overspeed with an increase of pressure just in the upstream of the turbine 7, to the upstream side of the exhaust brake valve 13, discharge flow quantity of air increases, and a brake increases being left as providing stable performance. That is, suppressing the overspeed, brake power increases.