1. A computer implemented particle size distribution model creating method of creating a particle size distribution model that simulates a particle size distribution of a cluster of particles comprising a plurality of particles having different particle sizes, the particle size distribution model creating method comprising: a particle size range determining step of determining, using a computer, a minimum particle size and a maximum particle size of the particles included in the cluster of particles to be simulated; an integrating step of integrating, using the computer, frequency of appearance of the particles within a particle size range determined in the particle size range determining step, using the minimum particle size as a starting point and the maximum particle size as an endpoint; a division point determining step of dividing, using the computer, an integration region obtained in the integrating step into a given number of regions by performing first division, using a value of integral of the frequency of appearance obtained in the integrating step, so that values of integrals obtained for the regions resulting from the first division are equal, and determining particle sizes at the ends of the regions as division points; a typical point determining step of determining the minimum particle size, the maximum particle size and the particle sizes of the division points, as typical points; a step of assuming a particle size distribution which contains particles having the particle sizes of the typical points, and is plotted such that frequencies of appearance of the particles having the particle sizes of the typical points are equal to the values of integrals obtained for the regions defined by the typical points; and a step of obtaining the assumed particle size distribution as a particle size distribution model.