To reduce piston surface pressure between the piston and cylindrical holes of a cylinder for hydrostatic transmission, to improve the torque efficiency of a machine driven by the transmission.

A cylinder block 28 is provided with a rotatable piston housing, plural piston cylindrical holes 32 arranged in a circular pattern in the housing, a pistons 34 which can slide the cylindrical holes respectively and have outer ends crossing over the opening end of the cylindrical holes to be extendable, receiving holes 36 having spherical bottom parts on the outer ends of the respective pistons, piston slippers 38 equipped with spherical base parts (balls) 40 for retaining in the receiving holes 36 and arms 42 extending from the balls 40, and having plans 46 extending in a lateral direction on the other ends, and rotation cam plates 48 having a flat control surface 54 interlocking so as to slide with the planes 46 of the arms 42.