A novel measuring method which allows measurement of fine particles which are extremely fine and contained in a very small amount in ultrapure water is provided. The number frequency with respect to the fine particles having a particle diameter of 0.1 m or more is measured based on the finding that the fine particles contained in the ultrapure water at the use point of an ultrapure water production system exhibits a logarithmic normal distribution to determine the logarithmic normal distribution of the fine particles contained in the ultrapure water, and the number of the fine particles having a particle diameter of less than those of the measured particles contained in the ultrapure water is obtained from the distribution.