A crankshaft of an engine is connected to a carrier shaft of a planetary gear via a damper. The damper includes a torsion member generating elastic force for suppressing relative rotation, when the crankshaft and the carrier shaft are relatively rotated. A control unit estimates engine torque with using rotation angular acceleration of the crankshaft calculated from a detected value of an engine speed sensor, and corrects the estimated engine torque with a correction term of the elastic force of the damper calculated based on a damper torsional angle. Then, the control unit diagnoses an abnormality of the engine based on the estimated engine torque after correction.