PURPOSE: To provide a simple method of purifying exhaust gas of rotary piston engine, which is capable of carrying out purification of exhaust gas in effective manner, by connecting a first and a second ternary catalysts to the exhaust system in series with each other along the exhaust gas flow, and controlling the temperature of these catalysts and the state of the exhaust gas introduced thereinto to enable effective purification of the exhaust gas.

CONSTITUTION: In view of the fact that a certain range of air-fuel ratio and a certain temperature range of catalysts are required for effective purification of CO, HC and NOx, respectively, a first ternary catalyst 11 and a second catalyst 12 are connected in series to exhaust pipe 10. Temperature of the exhaust gas introduced into the first and the second ternary catalysts is determined by the heat radiating area of the exhaust pipe 10 at the portion thereof from engine body 1 to the first catalyst 11 and from the first catalyst 11 to the second catalyst 12. CO and HC are purified mainly by the first catalyst 11, whereas NOx is purified mainly by the second catalyst 12