PURPOSE: To improve the position precision of a bypass hole in regard to a throttle valve and by this, to improve the fuel flow precision of a low speed fuel system.

CONSTITUTION: A body 30 is put in a carburetor main body 20, and an intake passage is formed by the connection of this body 30 inward passage 31 to a carburetor main body 20 inward passage 21. A bypass hole 25 of a low speed fuel system is piercingly provided from the inside of the passage 21 to (an arrow A direction) the outward of the carburetor main body 20 with the total closing position or the idle opening of a throttle valve 23 as a reference before the body 30 is put into the carburetor main body 20 and after the assembly of the throttle valve 23 as well. As a result, the position precision of the bypass hole 25 with regard to the throttle valve 23 is drastically improved.