PURPOSE: To make it possible to measure a turbidity degree by discriminating whether the turbidity is caused by earth and sand or plankton, by measuring the spectrum intensities of specimen water or filtered water resulting from the filtration of said specimen water.

CONSTITUTION: The scattering spectra of specimen water and filtered water are shown by the diagram and the spectrum 20 is the scattering spectrum of the filtered water while the spectrum 24a is the scattering spectrum of the specimen water containing earth and sand or plankton. The spectrum 24a has peaks in the vicinity of 600nm and 700nm and is different from the spectra 21aW23a of only kaoline, that is, earth and sand and, as a result, it is made known that the specimen water contains plankton other than earth and sand.