An HST housing axle driving apparatus for a small-sized running vehicle, such as a tractor, in which an axle casing is divided into a left-side half axle casing and a right-side half axle casing. In one of the left- and right-side half axle casings are disposed a hydraulic pump provided with a vertical pump shaft and a hydraulic motor provided with a horizontal motor shaft. In the other of the left- and right-side half axle casings are disposed a speed reduction shaft and a differential gear unit, so that a driving force is adapted to be transmitted from the vertical pump shaft to the horizontal motor shaft.