A welding apparatus for welding together overlapping ends of two lengths of thermoplastic synthetic resin sheets at a welding station comprises a wedge-shaped heating device for softening the thermoplastic synthetic resin along the overlapping sheet ends, the heating device being arranged ahead of the welding station in an operating direction and being displaceable in said direction. The apparatus has a frame including two parallel frame parts, a part connecting the two frame parts, the connecting part being rigidly affixed to one of the frame parts and the other frame part being pivotally mounted on the connecting part, and spring steel rods biasing the other frame part for pivoting towards the one frame part. Two oppositely disposed, motor-driven pressure rollers are pressable against each other, one of the rollers being mounted on the one frame part and the other roller being mounted on the other frame part.