Provided is an after-treatment apparatus for exhaust gas right after a combustion chamber, which apparatus comprises a discharge device with an electrode exposed to an exhaust port installed in a cylinder head, an antenna installed on the back face of a valve head, an electromagnetic wave transmission line installed in a valve stem with one end connected to the antenna and the other end, covered with an insulator or dielectric and extending to and connected to a power-receiving portion, which is positioned at a location fitting into the guide hole or at a location farther from the valve head in the valve stem, and an electromagnetic wave generator for feeding electromagnetic waves to the power-receiving portion. The after-treatment apparatus is configured such that discharge is generated with the electrode of the discharge device and electromagnetic waves fed from the electromagnetic wave generator through the electromagnetic wave transmission line are radiated from the antenna.