A self-locking T-piece connects an attachment to a breathing gas line. The self-locking T-piece has a line section (2), a connection line (4) branching off therefrom, a movable valve body (10) with a valve disk (12), and a valve seat (14). The valve body (10) is prestressed by a spring (16) into a position to seal the connection line (4) against the line section (2), with the valve disk in the valve seat, and moves out of the sealing position when connecting the attachment to the connection line. The valve body (10) is elastically deformable such that it can be advanced through the connection line (4) and the valve seat (14), while undergoing elastic deformation, and returns to its original shape during the entry of valve disk (12) into the line section, in which it has a sufficient extension to be able to cover the valve seat for sealing.
