PURPOSE: To provide a carburetor opening sensing device which is structured simple, allows suppression of the costs, can enhance the sensing accuracy, and permits widening of the degree of freedom in manufacture and also enhancement of the easiness in the works.

CONSTITUTION: An opening sensing device for a carburetor 1 is formed from a carburetor body 2 having a venturi passage 6 and valve chamber 7 and a variable area valve which varies the area of the venturi passage. A holder 11 to which a magnet 25 is fixed, is secured to the body 9 of the variable area valve, and magnetic sensors 28a-28c are installed in that position on the outer surface of a bulkhead 2d to form the valve chamber, which can be situated opposing to the magnet. The valve is composed of a valve body of piston type, a jet needle 21 inserted to the bottom wall of the valve body, and a valve spring 10 for pressing the needle 21 fast to the inner surface of the bottom wall, and the holder is made of resin inserted slidably in the valve body, and the needle is pressed by the holder fast to the bottom wall of the valve body.