PURPOSE: To prevent a heating wire from damage by providing a heating element inside a vortex generating object and detecting a current speed from a flow caused by Karman's vortex taking place in the vortex generation object.

CONSTITUTION: A pole type vortex generating object 13 provided with a vertical slit against the flow of fluid is set inside a pipe line 12. In the down stream of this vortex generating object 13, Karman's vortex 14 takes place the generating cycle of which is in proportion to the flow speed. When the fluid strikes the vortex generating object 13, the vortex 14 separates from its side face to go toward the down stream. Then, the pressure decreases by separation of the vortex 14 and the fluid in the slit moves to the vortex generating side. Since the vortex is generated alternately in both side, the current speed V of the fluid running across a heating wire 5 changes with the frequency proportional to the current speed in the pipeline. This change means the change in the resistance value of the heating wire 5, which is detected in a brige circuit, and the current speed is found.