To solve the problem wherein a hydraulic motor and an electric generator are separately formed in a conventional hybrid driving technique, and thereby a large arrangement space is required, a case for storing each device and a link mechanism between both the devices are required, compactification of an axle driving device is difficult, the number of parts is increased to increase parts cost and power transmission loss is large, and engine power is always main power and electric motor power is only auxiliary power in power for driving motor shafts, regardless of the size of engine load, and thereby fuel consumption and discharge gas can't be sufficiently reduced, with small saving energy effect.

Inner rotors 75 of the electric generators 34L and 34R are provided around the motor shafts 58L and 58R of the hydraulic motors 37L and 37R. The inner rotors 75 are connected to be integrally rotatable on the same axial center as those of the motor shafts 58L and 58R.

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