A power transmission system comprising: a power synthesizing mechanism connected in a power transmitting manner to an electric motor and a power generator; and a speed change mechanism for changing the rotating speed of the electric motor to transmit the motor torque to the power synthesizing mechanism. An arranging region for the electric motor and an arranging region for the power generator do not overlap on each other in the axial directions of the output shaft of the electric motor and the rotary shaft of the power generator.