If operation mode of an engine is transitioned from natural aspiration lean-burn operation mode to supercharged lean-burn operation mode, an engine control portion temporarily changes an air-fuel ratio from a lean air-fuel ratio to a stoichiometric air-fuel ratio or a rich air-fuel ratio. When the air-fuel ratio is changed from the lean air-fuel ratio to the stoichiometric air-fuel ratio or the rich air-fuel ratio by the engine control portion, a transmission control portion switches a gear ratio of the automatic transmission to a gear ratio that provides an engine rotational speed at which the engine output becomes equal to an engine output before the air-fuel ratio is changed and also at which an intake pipe pressure becomes equal to or closest to an intake pipe pressure before the air-fuel ratio is changed.