To make water leak deciding operation efficiency, using a pointer detection sensor fitted to a flowmeter and a data gathering device, and then automatically gathering data during the night time and processing them during the day time.

The sensor, which converts the rotating state of the flowmeter into a pulse signal and outputs the signal is fitted to the flowmeter and the output signal from the sensor, is gathered at specific sampling intervals day and night; and the minimum water distribution amount of a water supply pipe per unit time in a measurement time zone is found from the longest time of the absence of pulses from the sensor among gathered data and compared with a permissible water leak amount obtained, when the water supply pipe has no leakage, so that leakage occurrence of water is determined when the minimum water distribution amount is larger than the permissible water leakage amount.