NOVELTY - The machine has a piston (18) comprises an inner passage in which working fluid from the working chamber flows. A groove (60A, 60B) communicates with the inner passage on engagement surface (30) of piston engaging with a roller and to which working fluid is introduced via the inner passage from the working chamber. The groove comprises an annular groove portion (61A, 61B) and a communication groove portion (62A, 62B) traversing the annular groove portion along a rotation direction of roller in which the inner passage opens to the communication groove portion.    USE - Fluid working machine such as hydraulic pump and hydraulic motor of wind turbine generator (all claimed) installed at sea level as offshore wind turbine generator. Can also be used in other types of power generating apparatus of renewable energy type such as tidal generator, ocean current generator and river current generator.    ADVANTAGE - The communication groove portion is configured to traverse the annular groove portion and also along the rotation direction of the roller. Thus, the communication groove portion merges into the annular groove portion at more than one place. Therefore, the working fluid supplied from the inner passage is introduced to different portions of the annular groove portion via the communication groove portion, respectively. Working fluid supplied from the inner passage is introduced to each of the different portions of the annular groove portion via the communication groove portion, so that the working fluid can effectively spread throughout the groove. The working fluid is thus replenished to the groove effectively to form the hydrostatic oil pressure film in a stable manner. As a result, in the fluid working machine, the wear of the engagement surface having insufficient hydrodynamic effect can be reduced effectively by the hydrostatic effect, and also an extended lifetime can be achieved by supplying the fluid efficiently to the groove formed on the engagement surface of the piston engaging with the roller, with a simple structure.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a wind turbine generator.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the piston of wind turbine generator, in the radial direction of the ring cam.    Piston (18)    Engagement surface of piston (30)    Groove (60A,60B)    Annular groove portion (61A,61B)    Communication groove portion (62A,62B)