Excessive generation NO2 by oxidation catalyst arrangement upstream of a selective reduction catalyst is suppressed to prevent falling of NOx reduction rate.A selective reduction catalyst 4 capable of selectively reacting NOx with ammonia even in the presence of oxygen is incorporated in an exhaust pipe 3 from an engine 1. A pair of oxidation catalysts 5A and 5B are arranged in parallel with each other and upstream of the selective reduction catalyst. In an operation condition with low exhaust temperature, amounts of the exhaust gas 2 distributed to the oxidation catalysts 5A and 5B are adjusted so as to make NO/NO2 ratio in the exhaust gas 2 to about 1-1.5. In an operation condition with higher exhaust temperature where the NOx reduction rate is less affected by the NO/NO2 ratio, as a countermeasure against increasing, leaking ammonia, passage changeover is conducted to pass the exhaust gas 2 in backflow through the selective reduction catalyst 4 and the oxidation catalysts 5A and 5B, so that ammonia passing through the selective reduction catalyst 4 without reacted is oxidized in the oxidation catalysts 5A and 5B.