To prevent surface sink, deformation or the like of a piston part and to prevent the existence of damper action non-effective area caused by air reservoir, in an air damper using a piston made with a synthetic resin. Double annular recessed parts 9a, 9b as thinning parts are formed at a side opposite to a piston rod 4, of the piston 5 to prevent the surface sink or the like, and double annular walls 10a, 10b projecting into the annular recessed parts in a maximum recessed state of the piston, are integrally formed on a bottom face 3a of a cylinder 3. As the air accumulated in the recessed parts in the maximum recessed state of the piston is reduced, a free running distance (area where the damper effect can not be achieved) caused by the expansion of the air accumulated in the recessed parts can be reduced in starting the movement of the piston from the maximum immersive state, the damper action as the air damper can be exercised approximately over the whole stroke of the piston.