The invention describes an apparatus for welding a tubular lining of a wastewater pipe, comprising a retainer for the butt-jointed longitudinal edges (8, 9), which are intended to be welded, of a plastic panel (5) bent to form a pipe, and comprising a welding apparatus (25). In order to provide advantageous welding conditions it is proposed that the retainer for the two butt-jointed longitudinal edges (8, 9) of the plastic panel (5) comprises one respective clamping device (6, 7) consisting of a stationary clamping bar (10) on the outside of the pipe and a movable clamping bar (11) on the inside of the pipe which is connected to an actuating drive (12), that the stationary clamping bars (10) are located opposite of one another with respect to the butt joint between the two longitudinal edges (8, 9) while leaving a gap (21) open, that at least one of the two movable clamping bars (11) forms a stop (36) for one longitudinal edge (8) of the plastic panel (5) in a position placed against the associated stationary clamping bar (10), and that a guide (22) is provided along the gap (21) between the stationary clamping bars (10) for the welding device (25).