Abstract: A fastener driving tool prevents the tool from twice striking, so-called double-driving in a single driving operation. A portion closing the lower end of a cylinder of the tool is formed with an annular recess to receive a ring bumper as a shock absorber of a piston. The recess has a sufficient depth to move a lower portion of the bumper downward and upward. The bumper is fittingly pressed into the recess to form an air chamber in the recess closed by the lower end of the bumper. A check valve is positioned between the air chamber and a supply of the pressurized air to feed the pressurized air to the air chamber so that the pressurized air is further compressed by the bumper which is downwardly moved by the abrupt downward movement of the piston. Thus, greater reaction force is applied from the air chamber to the lower portion of the bumper so as to prevent a driver of the tool from the double-driving.