PURPOSE: To obtain a sensor which has superior resistance and sensitivity by transducing the pressure of air into AC vibration through a cavity and detecting the vibration by a vibration detector.

CONSTITUTION: The width 2 of a case 1 is reduced at the center part as compared with the entrance 3 and exit 4 on both sides so as to form a venturi pipe. A base 5, on the hand, closes the groove 2 to function as a flow passage 6. A cylindrical part 5 is formed in one body on the opposite surface from the flow passage 6 and the inside of the cylindrical part 5b is linked with the flow passage 6 through an opening 5a. A piezoelectric vibrator 7 is fixed at the top of the cylindrical part 5b by its node part. The cavity 8 is formed of the space surrounded with those base 5, cylindrical part 5b, and vibrator 7. When air is blown from the entrance 3 to the exit 4, the air in the cavity 8 is sucked through the opening 5a to receive air resistance, thereby vibrating naturally. The vibration is detected by the vibrator 7 to detect whether air is blown or not.