PURPOSE: To obtain a device which shows high fuel measuring accuracy and little temporal change and excellent durability in measuring of fuel of a sliding valve type carburetor.

CONSTITUTION: A jet needle 33 has an installation flange 35 on its upper side and a guide flange 36 on its lower side. A needle jet 18 has a mixing pipe 38 provided with a fuel passage concentrically, and a guide projection 40. A plurality of guide projections each having a diameter smaller than that of the fuel passage are partially projected toward the center on the fuel passage of the mixing pipe, along the flow passage. A space 41 is formed between the adjacent guide projections, while an air bleed hole 42 is formed in the space. The jet needle is arranged on a sliding valve 9 through an installation flange 9, while the guide flange is movably fitted in the guide projection of the mixing pipe. Thus, a measuring part 34 of the jet needle is centering-supported in a non-contact condition, while bleed air is mixed toward the fuel flowing the mixing pipe through the air bleed hole and the space.