NOVELTY - The machine has a set of pistons (24) arranged along a radial direction of the machine. A set of rollers (26) is provided rotatably for the set of pistons, respectively. A cam (32) includes a cam surface (33) contacting the set of rollers, and is rotatable around a rotation shaft of the machine relative to the set of rollers, where a polishing direction of the cam surface is different from a rotation direction of the rotation shaft, and a friction coefficient between the rollers and the cam surface is set larger than a friction coefficient between the roller and the piston.    USE - Radial piston type hydraulic machine for use in a hydraulic transmission of a wind turbine generator (all claimed). Can also be used for a tidal current power generating apparatus, ocean current power generating apparatus, river current power generating apparatus and a construction machine.    ADVANTAGE - The machine sets the polishing direction of the cam surface different from the rotation direction of the rotation shaft such that the lubricating oil is retained in minute grooves to retain oil film on the cam surface and maintain favorable lubricity between the cam surface and a roller, thus suppressing deterioration of the cam surface and improving cam life. The machine reduces surface roughness in a high pressure region with distribution in surface roughness of the cam surface, thus avoiding excessive designing, preventing size increase of the machine and improving reliability of the machine. The machine allows friction coefficient between the roller and the cam surface to be set larger than the friction coefficient between the roller and the piston to prevent skidding between the cam surface and the roller, thus preventing occurrence of seizure and smearing.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) a hydraulic transmission    (2) a wind turbine generator.    DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of a cam including a polishing direction of a cam surface.    Rotation axis (A)    Pistons (24)    Rollers (26)    Cam (32)    Cam surface (33)