PURPOSE: To enable rotational speed to be changed over in two steps by introducing oil holes to a hydraulic drive through a spindle while providing oil holes communicating selectively to the half number of cylinders from a spool body through a disk.

CONSTITUTION: A disk 25 is under the counterclockwise turned condition in low speed rotation and a slot 46 communicates to oil holes 23, 24 in a cylinder block 15 so that the high pressure side passing through an inflow hole 6 and an oil hole 11 and the return side passing through an oil hole 12 and an exhaust hole 7 communicate alternatively to 10 cylinders 16 and all pistons 17 rotate operatively under the normal condition. Next, in high speed operation, operativel oil is put into the small diameter portion of a stepped cylinder 61 to move rightward a spool 32 together with a piston 37 while turning a disk 25 integral with a projection 38 to shut off the oil hole 23. Thus, since the oil hole 24 communicates to an oil hole 47 and pressurized oil in the inflow port 6 flows into only the half number of cylinders 16 through an annular groove 36 and oil holes 53, 57, the supply amount of pressurized oil is doubled and thereby the rotational frequency is doubled.