To eliminate any influence on the fuel quantity in a high speed operation range caused by fuel quantity regulation in an idling operation range without changing the external configuration of an existing carburetor and/or mounting position of a fuel regulating needle valve.

Low speed fuel injection holes 24, 25 and an idle fuel injection hole 36 are opened in the vicinity of closing position of a butterfly type throttle valve in an air intake passage 10 and on the downstream side part from the throttle valve respectively. A constant pressure fuel chamber 14 is communicated with low speed fuel injection holes 24, 25 and the idle fuel injection hole 36 via a low speed jet 21 and a low speed fuel passage 22. A low speed fuel chamber 23 is provided in the low speed fuel passage 22 for communicating the constant pressure chamber 14 with fuel injection holes 24, 25 and an idle fuel regulation needle valve 32 and an idle fuel chamber 65 are provided in the low speed fuel passage 22 for communicating the constant pressure chamber 14 with the idle fuel injection hole 36. A space between the low speed fuel chamber 23 and a constant pressure chamber 14 and a space between the idle fuel chamber 36 and the constant pressure chamber 14 are blocked by a closing plate 62 and a closing plate 64 respectively.