On the basis of rainfall amount data (24) representing a timeseries change in the amount of rainfall in a district as an estimation target and unascertained water amount data (23) representing a time-series change in the amount of unascertained water at a base point located downstream of each district, an unascertained water occurrence distribution estimating unit (12) of a control unit (10) calculates a correlation value between the rainfall amount data (24) in each district and the unascertained water amount data (23), and outputs each correlation value as an unascertained water occurrence distribution in each district.