PURPOSE: To make the detecting speed of the change in flow rate quick, by providing a means for memorizing the period of a flow rate pulse signal and a means for detecting the elapse of time for every N times of the established period, computing the flow rate signal, and obtaining the flow rate signal 1/N the previous flow rate.

CONSTITUTION: The flow rate signal from a flow rate detecting part 1 is connected to a one-touch microcomputer 6 through a wave-form shaping circuit 5. The computer 6 is constituted of a pulse input port 7, a microprocessor 8, a clock generating circuit 9, a counter 10, an established-period memory part 21, a period comparison loop counter 22 and output interface 12. The elapse of time is detected by the comparison of the data from the counter 10 and the data from the period memory part 21. The flow rate signal 1/N the previous flow rate is obtained at the elapse of time that is N times the established period. Thus the change in flow rate can be detected without waiting the next flow rate pulse signal, and the stable measurement can be performed.