PURPOSE: To increase treating efficiency of the exhaust gas and to contrive the reduction of an electric consumption by providing a cylindrical electrode serving also as a path of introducing the exhaust gas and the needle type electrode arranged in the cylinder of the exhaust gas introducing path, provided separately with fixed distance from the relevant electrode and generating the plasma by applying voltage between the respective electrodes.

CONSTITUTION: In the case of activating an electric source device 6 and applying voltage between the electrodes 2, 5, corona discharge is generated between the electrodes. The corona discharge generated at this time constitutes bonnet type electric discharge path between all circumference of the cylindrical electrode 2 and the needle part 5a of the electrode 5. Since in the case of flowing the exhaust gas from the electrode 2 in a direction shown by arrow, the exhaust gas passes through relevant discharge path, namely passing through the atmosphere of the plasma discharge, the exhaust gas is treated with the decomposition and made flow into the exhausting pipe 3. By such a means, since exhaust gas passes through the plasma discharge, the decomposing efficiency is increased and the removal efficiency of NOx is increased. Also, by constituting the electrodes 2, 5, shifting to a spark discharge is suppressed and the electric power and the consumption of the power are also reduced.