A flywheel housing 13 is partitioned by a partition wall 14 disposed at an intermediate position in the lengthwise direction thereof to form a pair of housing portions in which a flywheel 13 is accommodated in one housing portion 13X, and an HST unit that receives power from the flywheel is installed in the other housing portion 13Y, the HST unit 5 comprising an oil hydraulic motor 9, an oil hydraulic pump 10, an oil hydraulic circuit block 17 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 18 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 17 being secured to the flywheel housing.