A biomass hydrothermal decomposition apparatus that feeds a solid biomass material 11 from one side of an apparatus body 42, feeds hot water 15 from the other side, to hydrothermally decompose the biomass material 11 while bringing the biomass material 11 into counter contact with the hot water 15, dissolves hot-water soluble fractions in hot water, discharges the hot water to outside from the one side of the apparatus body 42 as a hot-water effluent 16, and discharges a biomass solid (a hot water insoluble) 17 to outside from the other side. The biomass hydrothermal decomposition apparatus includes an effective reaction region A formed from the other side to the one side of the apparatus body 42, in which a feeding temperature of the hot water 15 (for example, 200° C.) is maintained for a predetermined period of time to cause hydrothermal decomposition, and a temperature drop region B in which a temperature is rapidly dropped to a temperature (for example, 140° C.) at which the hot-water soluble fractions are not excessively decomposed (for example, from 200° C. to 140° C.), immediately after it is out of the effective reaction region A.