An excessively large electric current is suppressed from flowing to an electrode. Provision is made for an electrode arranged in an exhaust passage of an internal combustion engine, a power supply connected to the electrode for applying a voltage thereto, an air fuel ratio detection device for detecting or estimating an air fuel ratio of an exhaust gas which flows through the exhaust passage, and a power upper limit setting device for setting an upper limit for electric power supplied to the electrode from the power supply in cases where the air fuel ratio detected or estimate.