To provide an inflow calculation program, apparatus and method for determining an appropriate inflow.

The inflow calculation apparatus 10 uses an inflow calculated for each of a plurality of calculation time intervals to calculate a flow difference between inflows of the time intervals. It is decided whether the calculated flow difference is not less than a predetermined threshold and being not less than the predetermined threshold (ΔQ) is continuing beyond predetermined time (ΔT). When it is decided that it is continuing beyond ΔT, an inflow having a short time interval out of the inflows causing the flow difference is selected as a qualified inflow.