1. A particle size model creating method of creating a particle size distribution model that simulates a particle size distribution of a cluster of particles of a catalyst metal of a fuel cell, said cluster of particles comprising a plurality of particles of the catalyst metal, comprising: determining a particle size range by determining a minimum particle size and a maximum particle size of the cluster of particles of the catalyst metal to be simulated; integrating the frequency of appearance of the particles in the determined particle size range, over an integration region defined by the minimum particle size as a starting point and the maximum particle size as an endpoint; dividing the integration region into a given number of regions through a first dividing operation, using the integral of the frequency of appearance, so that integrals obtained for the individual regions into which the integration region is divided are substantially equal, and determining particles sizes, of division points at which the integration region is divided; determining the minimum particle size, the maximum particle size and the particle sizes of the respective division points, as typical points; and obtaining a particle size distribution model by assuming a particle size distribution containing particles having the particle sizes of the respective typical points, said particle size distribution being plotted such that the frequency of appearance of the particles having the particle size of each of the typical points is equal to the integral obtained for each of the regions into which the integration region is divided at the typical points.