v 3 appearing on the output end of the operational amplifier 48 corresponds to the output which is derived only from karman 39 s vortexes the output end of the operational amplifier 48 is connected to the inverting input end of a voltage comparator 49 voltage vcc is applied to the non inverting input end of the voltage comparator 49 through a resistance 50 and the non inverting input end thereof is grounded through a resistance 51 a resistance 52 is connected between the non inverting input end and the output end of the voltage comparator 49 and further voltage vcc is applied to the output end thereof through a resistance 53 the voltage comparator 49 constitutes together with the resistances 50 53 a waveform shaping circuit converting the output v 3 of the operational amplifier 48 into pulse output v 4 on the other hand a circuit being composed of the resistances 65 66 67a is a circuit for outputting a sum signal v 5 of the output v 1 and the output v 2 and the value v 5 is the one responding to average flow velocity of fluid to be measured the output v 5 is applied to the non inverting input end of a voltage comparator 77 voltage vcc is impressed to the inverting input end of the.