PURPOSE: To improve the acceleration response of a turbo charger when an engine load rises up by controlling the rotational speed of a turbo charger to a predetermined rotational number in a range where deterioration of fuel cost is not caused upon the low-load drive of the engine.

CONSTITUTION: An inflow valve 16 for controlling the opening and closing of a passage is provided in the inlet part of an exhaust passage 14, and a bypass valve 17 for performing the opening and closing of the passage is provided in a bypass passage 15. In the above described construction, when the engine load is less than a predetermined value, a controller 22 operates so that the controller 22 supplies a relatively small quantity of exhaust gas to a turbine 8, and the rotational speed of the tarbine 8 is maintained to a predetermined value. By this operation, the engine load rises up, and when all the exhaust gas has been supplied to the turbine 8, the rotational speed of the turbine 8 rises quickly.