PURPOSE: To measure the particle having relatively high density easily by measuring the intensity at a first position relatively close to a start position for directing energy toward a fluid and similarly measuring the intensity at a relatively remote second position.

CONSTITUTION: When the density of particles suspended in a fluid is measured, a radiation energy is directed to the fluid and the intensity of energy scattered from the particles is measured at a first position relatively close to a start position for directing energy toward the fluid. Similarly, the intensity is measured at a second position relatively remote from the start position. As a measurement of density, artanh (Ef/En) is calculated (where, En is energy intensity at the relatively close first position, and Ef is energy intensity at the relatively remote second position). According to the method, relatively high density of particles can easily be measured.