Measurement of the particle size distribution of isopycnic floating particles using a solid-liquid fluidized bed was experimentally investigated. Experiments were conducted with a Glassbubbles-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. Despite incidences of spillage of small particles from the fluidized bed, there was a close match between particle size distributions measured by this measurement method and size distributions of particles remaining within the fluidized bed measured by the laser diffraction/scattering method. The ranges of particle sizes which can be measured are approximately 3 : 1 or 5 : 1 in terms of the ratio between the maximum and the minimum particle sizes. By controlling the solid-liquid fluidized bed in a state of uniform fluidization, the particle size distribution of floating particles can be measured.