When an engine power demand Pe* is not greater than a maximum power Pemax, the drive control sets a target rotation speed Ne* and a target torque Te* of an engine according to a reference operation curve obtained in air-fuel ratio control with a stoichiometric air-fuel ratio (step S125). When the engine power demand Pe* exceeds the maximum power Pemax, on the other hand, upon failure of a preset fuel increase prohibition condition, the drive control sets the target rotation speed Ne* to a maximum rotation speed Nemax (step S140) and sets a target air-fuel ratio used for the air-fuel ratio control of the engine to a rich air-fuel ratio based on the engine power demand Pe* (step S150).