In elevating the exhaust gas temperature for conducting the sulfur contamination-removing operation for a NO.sub.x occluding and reducing catalyst, a motor/generator is driven by the electric power from a battery while operating an internal combustion engine of a hybrid vehicle to produce a low output, and the wheels are driven via the internal combustion engine, power change-over mechanism and transmission. A motor assist is effected for increasing the output of the motor/generator during a period in the expansion stroke in each cylinder of the engine to increase the engine rotational speed during the above period only. Then, the exhaust valve is opened before the combustion is completed in the cylinder, and the exhaust gas temperature is raised.