A system for determining a number of particles comprising an exhaust gas introduction port to introduce an exhaust gas of an engine, a dilution gas introduction port to introduce a dilution gas, a main flow channel whose one end is connected to the exhaust gas introduction port, a dilution gas flow channel whose one end is connected to the dilution gas introduction port and whose other end is connected to the main flow channel, a diluter arranged at a connecting point of the main flow channel and the dilution gas flow channel or near downstream of the connecting point, a dilution gas flow rate control part that is arranged in the dilution gas flow channel and that controls a flow rate of the dilution gas introduced into the diluter, a particle number measuring unit, having a function of a constant flow rate, that is arranged downstream of the diluter through a first valve and that measures a number of solid particles in the diluted exhaust gas, a bypass flow channel that bifurcates from between the diluter and the particle number measuring unit in the main flow channel and in which a constant flow rate unit and a second valve are arranged, a suction pump that is connected to downstream of a converging point of the main flow channel and the bypass flow channel and that is to introduce the exhaust gas into the main flow channel and the bypass flow channel, and an information processing unit configured to receive (i) the dilution gas flow rate controlled by the dilution gas flow rate control part, (ii) a unit flow rate as being a flow rate flowing in the particle number measuring unit and (iii) a set flow rate of the constant flow rate unit in the bypass flow channel, and configured to calculate a dilution factor of the exhaust gas based on the dilution gas flow rate and a total of the unit flow rate and the set flow rate. an exhaust gas introduction port to introduce an exhaust gas of an engine, a dilution gas introduction port to introduce a dilution gas, a main flow channel whose one end is connected to the exhaust gas introduction port, a dilution gas flow channel whose one end is connected to the dilution gas introduction port and whose other end is connected to the main flow channel, a diluter arranged at a connecting point of the main flow channel and the dilution gas flow channel or near downstream of the connecting point, a dilution gas flow rate control part that is arranged in the dilution gas flow channel and that controls a flow rate of the dilution gas introduced into the diluter, a particle number measuring unit, having a function of a constant flow rate, that is arranged downstream of the diluter through a first valve and that measures a number of solid particles in the diluted exhaust gas, a bypass flow channel that bifurcates from between the diluter and the particle number measuring unit in the main flow channel and in which a constant flow rate unit and a second valve are arranged, a suction pump that is connected to downstream of a converging point of the main flow channel and the bypass flow channel and that is to introduce the exhaust gas into the main flow channel and the bypass flow channel, and an information processing unit configured to receive (i) the dilution gas flow rate controlled by the dilution gas flow rate control part, (ii) a unit flow rate as being a flow rate flowing in the particle number measuring unit and (iii) a set flow rate of the constant flow rate unit in the bypass flow channel, and configured to calculate a dilution factor of the exhaust gas based on the dilution gas flow rate and a total of the unit flow rate and the set flow rate.