PURPOSE: To provide a carburetor opening sensing device which can work well with smaller size of construction using a simple structure, allows suppression of the costs, accomplishes enhancement of the sensing accuracy, secures the space for fixation of a magnet, and prevents slipping off etc.
CONSTITUTION: An opening sensing device for a carburetor 1 is formed from 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, wherein a magnet 25 is secured to the valve 9 by means of insert molding. Magnetic sensor 28a-28c to emit the outputs in accordance with the intensity of the magnetic field generated by the magnet 25 are installed in 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.