A flywheel housing is partitioned by a partition wall disposed at an intermediate position in the lengthwise direction thereof to form a pair of housing portions in which a flywheel is accommodated in one housing portion, and an HST unit that receives power from the flywheel is installed in the other housing portion, the HST unit comprising an oil hydraulic motor, an oil hydraulic pump, an oil hydraulic circuit block containing an oil hydraulic circuit therein and supporting one end of an oil hydraulic motor shaft and one end of an oil hydraulic pump shaft, and a cover housing secured to the oil hydraulic circuit block to cover the oil hydraulic motor and the oil hydraulic pump and support the other end of the oil hydraulic motor shaft and the other end of the oil hydraulic pump shaft; the oil hydraulic circuit block being secured to the flywheel housing.