Particle size distributions of isopycnic floating particles were measured by use of a solid-liquid tapered fluidized bed. Experiments were conducted with a Glassbubbles K1-water system with a solid-liquid density ratio of 0.125, and a paraffin-24 wt% sodium chloride solution system with a solid-liquid density ratio of 0.760. Particle size distributions measured by the present method agreed with those measured by the laser diffraction/scattering method. The tapered shape of the fluidized bed suppresses the spilling of small particles. The ranges of particle sizes which can be measured are approximately 5 : 1 or 6 : 1 in terms of the ratio between the maximum and the minimum particle sizes. By controlling the solid-liquid tapered fluidized bed in a state of uniform fluidization, the particle size distribution of floating particles can be measured.