The minimum value for the degree of opening of a wastegate valve (24), wherein the flow volume of exhaust air flowing via a wastegate port (22) when the degree of opening of the wastegate valve (24) is changed toward the open side stops increasing, is defined as a first prescribed degree of opening. An electronic control device (50) controls the degree of opening of the wastegate valve (24) on the basis of the engine operating state, and when the vehicle is running steadily at a high speed, that is, when the degree of opening of the wastegate valve (24) which has been set on the basis of the engine operating state is equal to or greater than the first prescribed degree of opening, the degree of opening of the wastegate valve (24) is varied between values equal to or greater than the first prescribed degree of opening.