Disclosed is a waste gas treatment apparatus having a waste gas inlet for flowing a waste gas into a main combustion chamber provided in empty space inside a body, and a plasma torch configured to propagate flames against the waste gas flowing through the waste gas inlet. The apparatus comprises a steam injection nozzle configured to eject high temperature steam against the flames emitted through a nozzle of the plasma torch, and a reaction tube, extended longitudinally toward the bottom of the main combustion chamber into a tube shape, configured to perform the chemical reaction between a reaction accelerating compound and the waste gas induced by pressure of the nozzle of the plasma torch.