A biomass hydrothermal decomposition apparatus includes, a biomass feeder (31) that feeds biomass material (11) under normal pressure to under increased pressure, a hydrothermal decomposition device (42A) that allows the fed biomass material (11) to be gradually moved inside a device main body (42A) from either end thereof in a consolidated condition, and also allows hot compressed water (15) to be fed from an other end of a feed section for the biomass material into the main body (42A), so as to cause the biomass material (11) and the hot compressed water (15) to countercurrently contact with each other and undergo hydrothermal decomposition, and that elutes a lignin component and a hemicellulose component into the hot compressed water, so as to separate the lignin component and the hemicellulose component from the biomass material (11); and a biomass discharger (51) that discharges, from the side where the hot compressed water is fed into the device main body, a biomass solid residue (17) under increased pressure to under normal pressure.