NOVELTY - The peripheral surface end of high pressure projections opposing the inner periphery of cylinder (6), is coated with low friction materials such as ceramics. The gap (d) between the piston projection and cylinder inner peripheral surface is maintained to 0.03 mm.    USE - For lubrication free reciprocating type compressor to store hydrogen gas used e.g. in fuel cell system.    ADVANTAGE - Labyrinth-seal is achieved and the gas leak amount caused by sliding of the piston with respect to the cylinder is reduced. As the thickness of piston ring is reduced and mounting number of the piston ring is increased, reduction in size and cost of the apparatus is reduced and simplification of the process is achieved. Degree of freedom in design is enlarged and durability of apparatus is enhanced.    DETAILED DESCRIPTION - Several circular grooves (3) are provided in the piston (2) to engage with rings (4) of the cylinder. Low friction materials such as ceramics, polytetrafluoroethylene, polyimide and polyether ether ketone are coated to the peripheral surface end of the projections of pure material in the functional section of the piston. The inner peripheral surface of cylinder made of pure material is also coated with low friction material, in an alternate arrangement. Alternately the functional section of piston has high pressure and low pressure projections connected serially. The low pressure projection is made to diameter less than that of high pressure projection, which is made of iron or aluminum metal. Alternately, an endless contact piston ring is mounted to any groove at the terminal side of the low pressure projection side. A gas passageway is provided to connect the high and low tension sides of the endless contact piston ring, to produce moderate leak for pressure preservation maintenance.    DESCRIPTION OF DRAWING(S) - The figure shows the schematic sectional view of the piston-seal structure. (Drawing includes non-English language text).    Piston (2)    Circular grooves (3)    Rings (4)    Cylinder (6)    Gap (d)