To prevent or decrease generation of pre ignition and increase of combustion noise, while making the battery state of charge not to run short, when an engine driving a generator shifts from a lean operating condition to a rich operating condition for occlusion NOx purge.

A drive motor 40 is driven by the power generation electric power of a generator 20 driven by an engine 10 and the electric power from a battery 30. The engine 10 is made a rich operating condition from a lean operating condition at purge execution time. The power generation is continuously done while doing the EGR at a rich operating condition.