Abstract: In order to drift underground roads, tunnelling machines are used that have a rotatively driven boring head base body, a rigid first bit and rail heads that can radially sweep around axes of rotation between the working point in a conductor shaft and gallery faces by means of thrusting piston gears that carry at least one roller boring tool. In order to improve the coordination between an opening of the conductor shaft and under cutting, which are carried out simultaneously, a first bit. (2) is designed as a ring borer (2) that breaks out a drilling core (3) through slits of an annular cavity (4). The ring borer (2) has two parallel arms (6) spaced apart from a central axis (5) that carry boring tools (7) articulated on a cross-beam (10) of a carrying frame (9) that has an advancing device (11).