PURPOSE: To maintain tightness and prevent a drill from launching to incapability of functioning by forming a seal member of a bit shank slide part of a pneumatic hammering excavator from a deformable cylindrical film material, and fixing the ends to the periphery of the bit shank and the casing.

CONSTITUTION: A cylinder made of elastic material is folded back to form a seal 20, and one end 20A is fastened to a bit 22 by a stationary ring 60 while the other end 20B is pinched by the bottom of casing 10 and a cover 64. Then, pressure air is fed to an air hammer drill 14 from an air supply pipe 54, and a piston 28 is moved up and down to strike the bit 22, and thereby rocked is excavated. When the bit 22 moves up and down, one end 20A of the seal 20 is moved up and down, and the seal 20 deforms in following-up after this motion. Now the folded-back part 20C rolls up and down and moves. This prevents intrusion of muddy water to the contact surface of the seal 20 with bit 22.