A micro-traction drive unit according to the present invention includes an inner ring, an outer ring, a plurality of rolling elements, a retainer, a micro-traction drive case and a power-transmitting shaft coaxial with the inner ring and incapable of relative rotation around an axis to the retainer. An opposite end of the power-transmitting shaft from the inner ring forms an input end operatively connected to a driving source. An input end of the pump shaft is detachably connected to a central hole of the inner ring in a state incapable of relative rotation around the axis via the access opening of the micro-traction drive case by connecting the micro-traction drive case to the HST case so as to surround the input end of the pump shaft of the HST.