PURPOSE: To accelerate the updating of flow rate data and to enhance the detecting accuracy of the flow rate, by guiding the output signal of a Karman vortex sensor to a pair of period detecting means through a waveform shaping means, detecting the periods corresponding to the rise and fall of a rectangular signals, and computing the flow rate from the periods.

CONSTITUTION: The output signal with frequency corresponding to the flow rate from a Karman vortex sensor 31 in a sine wave shape is converted into a rectangular signal by a waveform shaping circuit 32. The signal is supplied to a rise-up detecting circuit 33 and a fall-down detecting circuit 34. Their output signals are supplied to interruption inputs of a CPU35. Counters 37 and 38 count the periods corresponding to the rise up and fall down of the rectangular waves, respectively. The period signals are inputted to the CPU35, where the flow rate is computed. Therefore, the updating of the flow rate data is accelerated, and the detecting accuracy of the flow rate is enhanced.