A power output apparatus 20 includes a clutch motor, an assist motor, and a controller. The clutch motor and the assist motor are controlled by the controller to enable the power output from an engine to a crankshaft 56, and expressed as the product of its revolving speed and torque, to be converted to the power expressed as the product of a revolving speed and a torque of a drive shaft and to be output to the drive shaft. The engine can be driven at an arbitrary driving point defined by a revolving speed and a torque, as long as the energy or power output to the crankshaft is identical. A desired driving point that attains the highest possible efficiency with respect to each amount of output energy is determined in advance. In order to allow the engine to be driven at the desired driving point, the controller controls the clutch motor and the assist motor as well as the fuel injection and the throttle valve position. Such control procedures of the power output apparatus enhance the energy efficiency of the whole power output apparatus.