To provide a free piston provided with structure capable of preventing deterioration of sealability due to twist of a seal ring when installing it into a cylinder.

The free piston 3 is provided with a piston main body 32, which is inserted into the cylinder 2 for housing freely to move inside the cylinder 2 and provided with a housing recessed part 31 in the periphery thereof, and the annular seal ring 33 to be installed in the housing recessed part 31 and to be brought in pressure-contact with the inner peripheral surface of the cylinder 2. A friction coefficient increasing processing 34 is performed to the housing recessed part 31 so that a friction coefficient of the housing recessed part 31 is higher than that of the inner peripheral surface of the cylinder 2.