To make an impact piston for performing an impact process of a pneumatically-operating impact tool abruptly exposed to this moving force for stressing the impact piston.

This pneumatic impact tool 1 includes: a cylinder pipe 2 closed by an impact plate 6 or a container wall and a cover 15; and the impact piston 9 movable in the cylinder pipe 2, wherein a first pressure chamber 27 is formed between the impact plate 6 and the impact piston 9 or between the container wall and the impact piston 9, the first pressure chamber 27 has a compressed air connecting part 26, a second pressure chamber is formed between the cover 15 and the impact piston 9, the second pressure chamber 28 a compressed air connecting part, the cover 15 has at least one opening 19 having a smaller area than the sectional area of the impact piston, and the impact piston 9 closes the opening 19 at a terminal position away from the impact plate 6 or the container.