Two-dimensional computer simulation was done to simulate the cross sectional view of non-woven fibrous filter, assuming that each fiber is arranged randomly and in parallel without overlapping other fibers. The distribution of the diameter of the inscribed circle drawn in the space formed by three adjacent fibers (D) and the distribution of thedistance between two fibers (L) were obtained by the Monte Carlo method. These size distributions evaluated by computer simulation were compared with the actual pore size distributions measured by the differential flow method (DFM) and microscopic observation of the cross sectional view of stainless steel fibrous filters. The following results were obtained, (1) The size distributions of D and L approximately followed the gamma (1) distribution, (2) The distribution of pore size measured by DFM (D-f) and by microscopic observation (L-M) followed the log-normal distribution within the limit range of distribution, but the size distribution of D-f also followed the Gamma distribution, (3) The relationship between the porosity of the filter (epsilon) and the median size of Df closely agreed with that of a vs. D, (4) The relationship between a and the mean flow pore size measured by DFM closely agreed with that of epsilon vs. the mean value of D at Gamma distribution, (5) The maximum value of D-f measured by bubble point test closely agreed with the maximum value of D and L at any porosity, (6) The relationship between a and the median size of L-M approximately agreed with that of epsilon vs. the median size of L.