Fluidized bed medium separation (FBMS) was applied to separate objects with a small specific gravity difference by precisely controlling the apparent specific gravity of gas-solid fluidized bed with binary particle systems. The binary systems of particles with various specific gravities and diameters were employed, and the apparent specific gravity of fluidized bed, g_fb and the dispersion, Δg_fb, were measured by changing the bulk volume fraction of heavier particle, V_hp, and the superficial air velocity, u₀. It was found that g_fb can be controlled by V_hp and u₀ because g_fb varies with the total mass of particles and the bed expansion. On the other hand, since Δg_fb is determined by the extent of segregation of fluidized particles and the fluidization intensity, Δg_fb decreases with reducing the extent of the segregation and the vertical fluctuation of fluidized bed surface. The experimental results showed that practical fluidized conditions for FBMS are in the range of g_fb≈0.5-3.8 and Δg_fb≤0.1. Under these fluidized conditions, plastics with a small difference in specific gravity (0.04-0.24) were almost completely separated at the top and the bottom of the fluidized bed.