To improve loading and unloading property of a cab by outputting the driving power energy from a first driving power energy means and the driving power energy reserved from a driving power accumulation means, converting that driving power energy to the rotation driving power by means of a second driving power conversion means, then outputting it to a drive shaft.

The rotation driving power of a drive shaft 59 is converted to hydraulic energy by means of a hydraulic pump motor 63 during braking of a truck, is output to an accumulator 43, and is temporarily stored. And, the hydraulic energy from a hydraulic pump 41 to be coupled with the rotary shaft of an engine 39 to be arranged horizontally rearward of a cab 37 during start acceleration and the hydraulic energy stored from the accumulator 43 are output, converted into the rotational driving power by means of the hydraulic motor 63, then output to the drive shaft 59, and transmitted to a driving wheel 55. Thereby, the loading and unloading property of the cab 37 can be significantly improved.