Decomposition behaviors of bamboo, chinquapin (hardwood), and Japan cedar (softwood) in hot-compressed water (HCW) were examined using a HCW flow type reactor to achieve the maximum utilization of plant biomass. HCW was passed through the reactor (3.6 mL) loaded with the biomass sample powder (177-250 mu m) at a flow rate of 10 mL/min under 9.8 MPa. The temperature of HCW was maintained at 180 degrees C for 20 min, after which it was raised to about 285 degrees C at a rate of 5 degrees C/min and maintained at 285 degrees C for about 7 min. By these operations, free sugars, some of lignin and most of hemicellulose, were first solubilized in water and flowed out during the initial 20 min flow of HCW. Cellulose started to decompose when the temperature of HCW was over 230 degrees C. The orders of elution of these components were common for three biomass samples, although the amount of products obtained during the heat-up period up to 180 degrees C and that of residue which finally remained in the reactor differed with the samples.