A power output apparatus including a power source, an electric motor and a connecting and disconnecting device, and a method of controlling the power output apparatus, are provided. When load driving force is applied from the power source to a power shaft in a condition where the power shaft and a driveshaft are disconnected from each other by the connecting and disconnecting device, lock control is executed to control the motor by fixing a direction of a magnetic field of a stator so as to restrict rotation of a rotor. When the rotor rotates during execution of the lock control, rotation retardation control is executed to control the motor by rotating the direction of the stator magnetic field in accordance with rotation of the rotor, so that driving force is applied from the motor to the power shaft in a direction opposite to that of the load driving force as driving force applied from the power source to the power shaft.