To provide an inflow calculation device capable of calculating the inflow of water flowing to a reservoir such as a dam at a calculation time interval agreeing with a predefined condition and preventing hunting when switching the calculation time interval.

In the inflow calculation device calculating the inflow by a time interval calculation method, the calculation time interval calculating the inflow is associated with the condition using the calculation time interval. When the inflow calculated by the calculation time interval does not agree with this condition, the inflow is calculated by another calculation time interval. When the calculated inflow agrees with the condition using the calculation time interval, the calculated inflow is made as the calculation result.

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