PURPOSE: To provide a diaphragm type downdraft carburetor which can operate under sudden acceleration just after restart after an engine stop.

CONSTITUTION: A main fuel supply line 30 is open at one end to a fuel regulator chamber 7 below a low-speed fuel jet 19 and is open to a venturi part V at the other end via a check valve 20. A fuel passage 31 upstream of the check valve and the passage 31 upstream of a main fuel jet are communicated with each other by a variable fuel controlling passage 33 having a variable flow rate control member 24.