PURPOSE: To reduce CO, HC, and NOx contained in exhaust gas by a procedure in which secondary air heated by exhaust gas is released into a carbureter and then gas supplied from the carbureter into an intake manifold is heated and expanded by said high-temperature secondary air for dilution.

CONSTITUTION: During the low-speed operation time of engine, fuel is sucked from an idle port into the intake pipe 12, and at the same time the suction negative pressure acts on the heated air jet flange 10, whereby the open air sucked up from the air cleaner 30 is heated through the air-fuel ratio regulator 35 and the heating device 20a heated by waste heat and then directed through the pipe 36', the peripheral groove 9, and the small hole group 8 into the intake pipe 12. The mixed gas passing through the carbureter with hot air is expanded by heating, vaporized completely, and then supplied into the cylinder 11. As a result, CO, NOx, and HC contents of the exhaust gas are suppressed to a minimum and therefore the rising of power and the saving of fuel during the low-speed time can be attained.