PURPOSE: To provide a carburetor opening sensing device which can work well with smaller size of construction through simplification of the sensing mechanism, allows suppression of the costs, and can avoid drop of the sensing accuracy by vibrations of its variable area valve.

CONSTITUTION: A carburetor is equipped with a carburetor body 2 having a venturi passage 6 and a valve chamber 7 formed in such a way as opening to the passage and a variable area valve 9 arranged in the valve chamber 7 slidably and varying the area of the venturi passage, and an opening sensing device is formed for this carburetor, wherein a magnet 25 is secured to the valve 9, and a magnetic sensor 28c, etc., to emit the outputs in accordance with the intensity of the magnetic field generated by the magnet 25 is fixed to that position on the outer surface of a bulkhead 2d to form the valve chamber 7 which can be situated opposingly to the magnet 25, and the magnet 25 and magnetic sensor 28c are arranged as opposing on a straight line approx. perpendicular to the suction gas flowing direction when viewed in the valve sliding direction.