A biomass hydrothermal decomposition apparatus that feeds a solid biomass material including cellulose and hemicellulose from one side of an apparatus body, feeds pressurized hot water from the other side, to hydrothermally decompose the biomass material while bringing the biomass material into counter contact with the pressurized hot water in the apparatus body, dissolves hot-water soluble fractions into hot water, discharges the pressurized hot water to outside from the one side of the apparatus body, and discharges the biomass material to outside from the other side, the biomass hydrothermal decomposition apparatus comprising:
- an internal-temperature cooling unit that rapidly drops a temperature after performing hydrothermal decomposition for a certain period of time;
- a temperature measuring unit that measures an internal temperature;
- and
- a controller that controls an internal temperature to be maintained at a predetermined cooling temperature by the internal-temperature cooling unit based on a temperature measurement result obtained by the temperature measuring unit.