A rainwater inflow facility management supporting method performs long-term, stable and accurate estimation of a flow rate change or water quality change in a rainwater inflow facility. The method which estimates the flow rate or water quality of sewage water from electric conductivity in the sewage water in the rainwater inflow facility is characterized in that the method calculates the a dilution ratio by dividing the electric conductivity value of sewage water on a clear weather day observed by a conductivity meter (12) installed at the rainwater inflow facility by the electric conductivity value of sewage water on a rainy day (with rainwater mixed therein) observed by the conductivity meter (12), and estimates the flow rate of the sewage water according to the dilution ratio.
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