PURPOSE: To provide a speed regulator for carburetor which reduces control man-hour for obtaining low speed air quantity, and improves the development efficiency of a carburetor, and moreover has high durability against foreign matter.

CONSTITUTION: The first low speed air passage 30 opened to the air is opened into a fuel well. The second low speed air jet 31 is arranged in the upstream of the first low speed air passage 30, and the first low speed air jet 32 is arranged in its downstream. The jet diameter of the first low speed air jet 32 is selected larger than the jet diameter of the second low speed air jet 31. The second low speed air passage 33 located between the first low speed air jet 32 and the second low speed air jet 31 is opened toward the air, and a normally open type opening and closing valve V to close the second low speed air passage 33 in the case where negative pressure in a suction passage 2A on a side from the throttle valve 3 toward an engine exceeds a fixed value is arranged in the second low speed air passage 33.