PURPOSE: To obtain a diffusional dilution system for measuring the quantity of particles contained in a fluid (reactive gas) accurately in safety and a method for measuring the particles contained in the fluid using the system.

CONSTITUTION: The diffusional dilution system comprises a laminar flow tube 10 having a length sufficient for diluting a fluid 22 containing particles and a fluid 18 for dilution under a state where the fluids form a laminar flow coaxially in the longitudinal direction, and an ejection tube 24 disposed coaxially with the laminar flow tube at the inlet end thereof, while being coupled with a particle containing flow source, so that an ejected fluid containing particles flows through the laminar flow tube 10 from the longitudinal inlet end 12 to the outlet end 14. The diffusional dilution system further comprises a capturing tube 34 disposed coaxially with the laminar flow tube at the outlet end thereof in order to capture the diluted fluid containing particles at the outlet end of the laminar flow tube, and a diluting fluid introduction chamber 20 juxtaposed to the ejection tube while sealing the inlet end of the laminar flow tube and coupled with a diluting fluid source provided to supply a fluid while covering the fluid containing particles ejected from the ejection tube coaxially.