When opening or closing an exhaust cut valve in accordance with an operation range of an engine and performing feedback control for an opening degree of a wastegate valve such that a target supercharging pressure is achieved in each of an open operation range where the exhaust cut valve is opened and a closed operation range where the exhaust cut valve is closed, a failure of the exhaust cut valve can be accurately determined. Whether or not an abnormal state occurs is determined in each range. In the abnormal state, although feedback control for closing the wastegate valve is performed, the actual supercharging pressure is lower than the target supercharging pressure, and a deviation between the actual supercharging pressure and the target supercharging pressure is maintained at not less than a predetermined value. Further, whether the exhaust cut valve is normal or in failure is determined.