A welding apparatus for welding flat webs or web sections made of synthetic thermoplastics comprises two parallel welding bars, which are adapted to be raised and lowered relative to each other, and tensioned strips of polytetrafluoroethylene, preferably strips of polytetrafluoroethylene which are reinforced by glass fiber woven fabrics, which strips cover said welding bars and are fixed on rolls, which are disposed beyond opposite ends of each welding bar and serve to wind up and unwind said tensioned strips. The rolls are movably mounted beyond opposite ends of the welding bars on axles extending transversely to said welding bars.