Aluminium-piston containing 12-25% Silicon, the surface of which is oxidised by microarc-oxidation

A piston (1) has a piston body (10) made of a material selected from any one of aluminum, an aluminum alloy, magnesium, and a magnesium alloy containing 12-25% silicon. An entire surface or a predetermined portion of the surface of the piston body (10) is covered with a plasma oxide film (17) by a microarc-oxidation process.