Abstract: A high frequency welding device for welding plastic sheets, in which the high frequency energy is provided by fixed-frequency oscillator, preferably by using a quartz crystal amplifier, whose frequency does not change during the welding process. The output circuit is designed to have a resonance curve which, before the occurence of any change in the output and coupling circuit characteristics due to the welding process, intersects with the fixed frequency of the oscillator at a "operating point" which is on the slope of the resonance curve descending to lower frequencies. The circuit components are selected to behave in such a manner that the resonance curve undergoes a shift toward lower frequencies, with the fixed-frequency "operating point" rising across the peak of the resonance curve to a point on the slope of the curve descending to lower frequencies, as the welding process progresses.