1. A spatial simulation program for controlling a computer to reproduce a time-based change of a phenomenon represented by a variable in a space represented by the plurality of numerical grids, comprising: defining a sub domain composed of the plurality of numerical grids; setting a forcing condition for the sub domain; detecting the sub domain including the numerical grid in which the variable changes with time; detecting the sub domain including the numerical grid in which the variable referred to for calculating the time-based change is positioned; and updating variables in the numerical grids constituting the detected sub domain.