1. A computer readable medium recorded with 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 plural sub domains, wherein a plurality of numerical grids neighboring each other constitute each sub-domain, and wherein each numerical grid of the numerical grids holds one or more variables to represent the phenomenon within the numerical grid, and the numerical grid is initialized with spatial data; setting a condition for updating the one or more variables associated with a subject numerical grid, and registering identification of subject sub-domains including the subject numerical grid, into an update-required table; detecting sub domains including at least one numerical grid in which any value of the one or more variables changes with time, by checking, for subject sub-domains registered in the update-required table, whether a first numerical grid included in a first sub-domain registered in the update-required table, is needed to be referred to when a numerical model updates the one or more variables associated with a second numerical grid included in a second sub-domain which is not registered in the update-required table, wherein the numerical model includes equations which change values of the one or more variables associated with a handled numerical grid by referring to the variables associated with the handled numerical grid and its neighboring numerical grids, and the first numerical grid is needed to be undated based on a judgment condition which includes equations which return a value indicating whether the first numerical grid is needed to be undated based on the variables of the first numerical grid, and registering identification of the second sub-domain into the update-required table; updating the one or more variables in the numerical grids constituting the detected sub domains, by applying the numerical model for the numerical grids in the sub-domains registered in the update-required table; and outputting values of the one or more variables of the numerical grids in the sub-domains registered in the update-required table, onward to memory.