The invention relates to a burner as well as to a waste gas treatment device for generating a flame for the combustion of process gas, especially of contaminants, in a combustion chamber, in each case having feed lines for a fuel gas and for an oxidizing agent so that they flow into a pre-mixing chamber, and having an ignition device for igniting the gas mixture contained in the pre-mixing chamber. According to the invention, a sensor for detecting and/or monitoring the flame is provided on the burner, especially at one end of the burner situated opposite from the pre-mixing chamber.