A system for determining a number of particles includes a diluter arranged at a connecting point of a main flow channel and dilution gas flow channel, a dilution gas flow rate control part that controls the flow rate of the dilution gas introduced into the diluter, a particle number measuring unit that measures a number of solid particles in a diluted exhaust gas, a bypass flow channel that bifurcates from between the diluter and particle number measuring unit in the main flow channel and in which a constant flow rate unit is arranged, and an information processing unit that calculates the dilution factor of the exhaust gas based on a dilution gas flow rate controlled by the dilution gas flow rate control part, and a total of a unit flow rate of the particle number measuring unit and a set flow rate of the constant flow rate unit.