A generator use gear enable to rotate freely against an input shaft of a transmission mechanism for altering a rotation number of an engine and a clutch mechanisms in which the input shaft of the transmission mechanism and the generator use gear are rotated together with are provided between the engine and the transmission mechanism. Further, on an extension line of the input of the transmission mechanism and to a rear portion of the transmission mechanism, an electric motor is provided. Accordingly, a compact size power transmission system of an automobile can be attained.