bores and move in reciprocation through biasing springs a movable swash plate 11 having a thrust bearing 11 a abuts against the heads of pistons 12 at the center of movable swash plate 11 is formed an opening 11 b through which pump shaft 3 perforates pump shaft 3 also serves as an input shaft and is disposed along the rotational axis of cylinder block 16 and is not relatively rotatably retained thereto the upper end of pump shaft 3 projects outwardly from the upper wall of upper half housing 1 and fixedly supports an input pulley 43 having a cooling fan 44 input pulley 43 is given power from a prime mover not shown of the vehicle to which the axle driving system is mounted through a belt transmission mechanism also not shown the piston abutting surface of movable swash plate 11 is desirably slantingly movable from a horizontal state with respect to the rotational axis of cylinder block 16 thereby enabling the amount and direction of discharged oil from the hydraulic pump to be changed the rear surface of movable swash plate 11 is convex and the inner surface of a lid member 15 fixed to upper half housing 1 which closes an opening in the upper wall is made concave to match with the convex rear surface of movable swash plate 11 movable swash plate 11 is constructed to be of a cradle type which when slantingly moved slides while coming into close contact with the concave surface of upper half housing 1 in order to slantingly operate movable swash plate 11 as shown in fig1 and 3 a control.