To provide a technique for detecting unknown water without requiring a flow meter.SOLUTION: An unknown water detection device of an aspect of the present invention includes: a feature quantity extraction unit that extracts an acoustic feature quantity pattern indicating a time change of the acoustic feature quantity from acoustic data including a flowing water sound; and an unknown water prediction unit that predicts whether there is unknown water from the acoustic feature quantity pattern of the acoustic data of a prediction object by using a machine learning model learned with the acoustic feature quantity pattern extracted from data including one or both of acoustic data including a flowing water sound when there is no rainfall and acoustic data of a flowing water sound under a different condition from when there is no rainfall.SELECTED DRAWING: Figure 5
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