This biomass treatment system includes: a hydrothermal decomposition section for decomposing the cellulose, hemicellulose and lignin contained in a biomass as a raw material under high-temperature and high-pressure conditions in a main section, namely a tank having a gas-liquid interface, and thus removing a lignin component and a hemicellulose component from the biomass; a discharge section for discharging a biomass solid (a component insoluble in hot-water) obtained by the decomposition; a slurrying tank which is connected to the discharge section and in which the discharged biomass solid is slurried in water fed thereinto to form a biomass slurry; and a solid-liquid separation apparatus including both a settling tank which is provided on a discharge line for discharging the biomass slurry and in which the biomass slurry is settled and a scooping-up and conveying means for scooping up the biomass solid deposited at the bottom of the settling tank and separating the solid biomass component from water.