To provide a good sliding characteristic in which there is no sticking phenomenon of an aluminum made cylinder by forming a carbon made piston ring of a carbon material formed by impregnating resin on a carbon base material setting an open porosity to a specified value.

In a place having a high humidity, there is such a trend that corrosion caused by galvanic action is generated by water drops which is in dew condensation in inner surfaces of a carbon made piston and an aluminum made cylinder. When the corrosion is developed, the piston ring and the cylinder are sticked to each other. The piston ring is formed of a carbon material formed by impregnating resin on a carbon base material whose open porosity is not more than 15%, and thereby, the carbon made piston ring is prevented from sticking to the aluminum made cylinder. In an air type braking device for a sliding door closer, the piston ring is interposed between an aluminum pipe 5 arranged on an outside as a cylinder and an aluminum pipe 4 arranged on an inside as a piston rod, and is suitably used as a piston ring 1 fixed to the aluminum pipe 4 arranged on an inside.