To provide a vehicular motor assist fitting structure, and a fitting method thereof in which hybrid vehicle using both an engine and a transmission of an existing gasoline engine vehicle is realized, and a motor case can be easily fixed to an engine even when an intense magnetic force is present when a motor rotor is inserted in a motor stator.

An engine side bolt fitting portion is provided on the engine, a motor side bolt through hole is made in the axial direction of the motor case corresponding to the engine side bolt fitting part, a transmission side bolt fitting portion is provided on a transmission case in the axial direction corresponding to the motor side bolt through hole, a stud bolt which is fitted to the transmission side bolt fitting portion and fitted to the engine side bolt fitting portion through the motor side bolt through hole is provided, the stud bolt is provided with a bolt side stepped portion having the outside diameter larger than that of other portions in a portion in contact with the motor case when fixing the motor case and the transmission case to the engine, and the motor side bolt through hole of the motor case is provided with a hole side stepped portion having the inside diameter larger than that of the other portions corresponding to the bolt side stepped portion of the stud bolt.