Abstract: An impact device comprises a cylindrical casing accommodating a hammer piston imparting blows to a working tool, which reciprocates under the action of compressed air admitted from a compressed air line through a control valve air distribution arrangement and a mechanism for automatically starting the device. The mechanism for automatically starting the device comprises a hollow two-step cylinder secured to the cylindrical casing accommodating a two-step piston which incorporates a valve co-operating with a tappet. The two-step piston is mounted in the hollow two-step cylinder in such a manner that an annular chamber is defined between the outer surface of the piston and the inner surface of the cylinder, the piston co-operating with the valve.