When engine is in the lean operation, and a WGV is fully closed, a boundary between a stoichiometric region and a lean region is corrected by learning using a turbocharging pressure obtained from a turbocharging pressure sensor. When the engine is in the lean operation, and the WGV is fully closed, the turbocharging pressure obtained from the turbocharging pressure sensor is equal to the actual maximum value of the engine torque output when a throttle valve is fully opened and the engine is in the lean operation, that is, the turbocharging pressure required to achieve the torque on the actual boundary. Therefore, the boundary can be brought close to the actual boundary by learning using the turbocharging pressure obtained from the turbocharging pressure sensor when the engine is in the lean operation and the WGV is fully closed.