To prevent energy regeneration depending upon the drivers' demand from being disabled by an insufficient internal pressure of an accumulator where energy recovery accumulates pressure.

During brakeage, variable displacement hydraulic pump motors 3 and 4 interlocking with the rotation of driven wheels 1 and 2 convert vehicle deceleration energy into oil pressure and pass it to an accumulator 11 for energy recovery. The variable displacement hydraulic pump motors 3 and 4, except during brakeage, on the other, convert oil pressure accumulated in the accumulator 11 into drive energy and use it in driving the driven wheels 1 and 2 for energy regeneration, and at the same time force a controller 16 to decide the internal pressure of the accumulator 11 is below a level sufficient for the energy regeneration, whereupon an electric motor 14 is actuated to feed oil pressure from a hydraulic pump 13 to the accumulator 11 so that the internal pressure of the accumulator 11 is kept not falling below the level sufficient for the energy regeneration.