An apparatus for welding a tubular lining of a wastewater pipe includes a retainer for the butt-jointed longitudinal edges, which are to be welded, of a plastic panel bent to form a pipe, and a welding apparatus. The retainer includes one respective clamping device for each edge formed by a stationary clamping bar on the outside of the pipe and a movable clamping bar on the inside of the pipe which is connected to an actuating drive. The stationary clamping bars are located opposite of one another with respect to the butt joint between the two longitudinal edges while leaving a gap open. At least one of the two movable clamping bars forms a stop for one longitudinal edge of the plastic panel in a position placed against the associated stationary clamping bar. A guide is provided along the gap between the stationary clamping bars for the welding device.