Abstract: A threshold value is stored in a CPU using setting switches. When a fluid is introduced into a flow passage, a rod disposed in the flow passage sheds Karman vortexes downstream thereof in the fluid as it flows through the flow passage. A piezoelectric device generates an electric signal depending on pressure fluctuations of the Karman vortexes, and the electric signal is applied through a comparator to the CPU. The CPU refers to a data table stored in a memory to determine a flow rate of the fluid based on the period of the electric signal. When the determined flow rate is greater than the threshold value, the CPU turns on transistors to energize loads as external devices. When the determined flow rate is smaller than the threshold value, the CPU turns off transistors to de-energize loads.