PURPOSE: To improve the accuracy of detection in a micro flow rate region by cutting off the frequency components above the frequency at which Karman vortexes are generated in the micro flow range among the output components of a vortex detector provided on the downstream of a vortex generator.

CONSTITUTION: A comparator 19 compares the output from an electric power amplifier 7 and a prescribed reference voltage, and opens or closes a switch 20 connected in parallel with a resistor 16. If the output from the amplifier 7 is lower than the comparing voltage, that is, in a micro flow rate range, the output of the comparator 19 is at a zero level and the switch 20 is opened; therefore, a low-pass filter 5 keeps functioning. When the output from the amplifier 7 rises to the comparing voltage or above, that is, in the stage of a high flow rate, the comparator 19 closes the switch 20 to short circuit both terminals of the resistor 16; therefore, the filter 15 does not function and the output signal of the amplifier 7 passes through the same as it is.