To purify NOx by reduction efficiently under both temperature up and down conditions by utilizing hydrocarbon in the exhausts of an oxygen excess ambience.

This device comprises a partial oxidation catalyst 2 partially oxidizing hydrocarbon in the exhaust gas, a temperature detecting means 4 to detect a temperature in this partial oxidation catalyst, a storage means 52 wherein relations between catalytic activity and temperature of the partial oxidation catalyst are stored, a temperature adjusting means 6 adjusting temperature of the partial oxidation catalyst, a control means 5 making the temperature of the partial oxidation catalyst into the specified temperature by driving the temperature adjusting means after comparing an actual temperature in the partial oxidation catalyst and the relation stored in the storage means, and a main catalyst 3 for deoxidizing nitrogen oxide into catalytic reduction. Since the temperature of the partial oxidation catalyst 2 is optimally controllable, those highly active HC and NOx being partially oxidized react on each other, whereby NOx catalytic conversion efficiency is improved.