PURPOSE: To improve start and warm-up performance of an internal combustion engine, in which a carburetor is equipped in an intake pipe, by such constitution that a control valve is provided in a bypass passage and opened at start and warm-up operation of the engine to allow a mixture to pass through catalytic converters heated with a heating unit.

CONSTITUTION: A series of bypass passages 6 are formed in a carburetor main unit 1 and heating cylinder H by detourting a throttle valve 5, and opened with an inlet 6a to the upstream part of a suction path 2 further opened with an outlet to a suction path Ha of the heating cylinder H, respectively. Then a fuel nozzle 8, communicated to under an oil level in a float chamber through a fuel jet 7, is opened to the upstream part of the bypass passage 6 passing through a carburetor C, and a control valve 9 is provided to the opening part. Further a heating chamber 11 is formed in the downstream part, passing through the heating cylinder H, of the bypass passage 6, here upper and lower two-stage catalytic converters 12, 13 are arranged, further a heating unit 17 is arranged to a lower part chamber 16 of the converter and connected to a battery 20 through a switch 19. In this way, a mixture can be effectively heated when an engine is started and warmed up.