PURPOSE: To perform improvement of the purifying efficiency of exhaust gas by effecting the periodic feed of heated secondary air to a position situated upper stream from a catalyst device.

CONSTITUTION: A bypass valve 12 is disposed between a conduit 9 and first and second air introduction pipes 10 and 11 and a heater 13 is located in the route of the first air introduction pipe 10. In which case, by effecting periodic switching of the bypass valve 12 by a control device 19, secondary air introduced through the conduit 9 with the aid of an air pump 8 is caused to flow in alternately the first and second air introduction pipes 10 and 11. As a result, heated secondary air is periodically introduced to a position situated upper stream from a catalyst device 5. Exhaust gas is mixed with the introduced secondary air and the mixture is introduced to the catalyst device 5 as the mixture is fluctuated between a rich and a lean state of oxygen.