The size of particles is detected accurately and at low cost even when there are few microparticles as impurities included in a liquid. Provided is a method of detecting a size of particles in a liquid by detecting diffraction fringes appearing due to the particles in the liquid by a light detection portion. Diffraction fringes are detected by a first light detection portion and a second light detection portion that are separated along the flow direction of the liquid. A peak time difference (T2) that is a difference between times at which peak values appear in a detection signal from the first and second light detection portion is measured, and an area (SQ) based on the waveform of the detection signal is measured. The sizes of the particles included in the liquid are detected based on the peak time differences (T2) and the areas (SQ) that were measured.