When simulating behavior of particles in a space having regions and subject to analysis, by processes of a predetermined number fewer than the number of regions, a particle simulation apparatus assigns the regions of the predetermined number, which are selected in descending order of the number of particles included in each of the regions, to differing processes among the processes of the predetermined number. The particle simulation apparatus sequentially assigns unassigned regions in descending order of the number of particles included in each of the unassigned regions, to the process for which the number of particles included in the regions already assigned to each of the processes of the predetermined number is the smallest.