To detect the size of a particle accurately and inexpensively even if the number of particulates which are impurities included in a liquid is very small.

In this method for detecting size of particle by applying coherent light to cross the direction of flow of the liquid flowing in a pipeline having light transmission property and detecting diffraction stripes AM caused by the particles included in the liquid by light detection means SE, the first and second light detection means SE1, SE2 are provided by separating them from each other by only the predetermined distance; a peak time difference T2 being difference in time when detection levels become the maximum levels, respectively, and a passing time T1 being the time from the start of detection by the first light detection means to the end of detection by the second light detection means are measured with respect to the diffraction stripes AM detected by these light detection means, and size of the particle included in the liquid is detected based on the measured peak time difference T2 and the passing time T1.