PURPOSE: To measure an intake air flow accurately by providing a Karman vortex street flowmeter with a means which generates a turning air current for reverse flow and a means which extinguishes this current.

CONSTITUTION: The number of vortex streets which are proportional to the quantity of air flowed from an air flow-in hole 1 are generated by a vortex generating column. When generated vortex streets pass between an ultrasonic transmitter 4 and an ultrasonic receiver 3, the ultrasonic receiving waveform is modulated and is processed by a separately provided electric circuit and is detected as the number of vortex streets. Next, air is sucked into an internal combustion engine through a connecting hole 5, a vortex chamber 10, gaps of guide blades 6, an exit tube 8, and a throttle valve 7. At this time, vortex streets are not generated in the vortex chamber 10. when the air current blown back from the internal combustion engine enters into the vortex chamber 10 through the exit tube 8 from the throttle valve 7, vortex streets are generated by the action of guide blades 6, and the retained energy is extinguished. When vortex streets are flowed into the connecting hole 5, they disappear by rectification due to the action of a rectifying grid 9, and at this time, the energy retained in the air current is extinguished also.