NOVELTY - The machine has a piston formed with a stepped portion (31) between a roller holding part (28) and a chamber forming part (27) such that a roller holding part is formed with larger diameter than the chamber forming part. The roller holding part holds the roller such that a rotational axis of the roller is parallel to a rotational axis of a ring cam. The chamber forming part includes a piston ring groove (29), which is formed on an outer periphery of the chamber forming part on a working chamber side of the stepped portion. A piston ring is provided in the piston ring groove.    USE - Fluid working machine e.g. hydraulic pump and hydraulic motor, for a wind turbine generator (claimed) i.e. renewable energy type wind turbine generator (claimed), to generate electric power from wind as a form of renewable energy. Can also be used for a tidal generator, an ocean current generator and a river current generator.    ADVANTAGE - The roller holding part is formed with the larger diameter than the chamber forming part to enhance rigidity of the roller holding part holding the roller, which moves in response to rotation of the cam, thus improving durability of the piston and lowering surface pressure between the roller and the piston. The chamber forming part is formed with smaller diameter than the roller holding part, thus improving freedom of arranging component parts and inner passages of the fluid working machine to easily create a design with space around the working chamber to arrange a valve mechanism for supplying and discharging the fluid to the working chamber. The roller holding part is formed such that working fluid is supplied to an engagement surface via an inner passage from a working chamber and working fluid for lubricating the engagement surface is drained, thus improving lubrication effect on the engagement surface. The piston ring is provided for purpose of suppressing leaking of the working fluid in the working chamber and provided in the piston ring groove formed in the outer periphery of the chamber forming part on the side nearer to the chamber than the stepped portion such that the stepped portion is not placed inside the working chamber and not subjected to the working fluid filling the working chamber, so that the stepped portion does not interfere reciprocating motion of the piston in a cylinder, thus achieving superior operational efficiency of the machine, and hence enhancing lifetime of the machine.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a wind turbine generator.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of a piston.    Chamber forming part (27)    Roller holding part (28)    Piston ring groove (29)    Engagement surface (30)    Stepped portion (31)