An apparatus and method for treating exhaust gases. A plurality of stages of reactor chambers are connected in series in the direction of an exhaust gas flow. High-voltage power supplies are connected to the reactor chambers In each of these reactor chambers, a streamer discharger plasma is generated. The more downstream a reactor chamber of a stage is placed, the lower energy to be cast into the reactor chamber becomes. The density of electrons generated in a gas decomposition unit is higher on the upstream side of the exhaust gas flow and the electron density is lower on the downstream side. A pulse generator is provided in which a high voltage, which is an output voltage of a D.C. charger (V0), is simultaneously applied to a plurality of distributed constant lines, which are connected in parallel with one another, by a signal shortcircuit switch (S1).