We have developed a flow reactor system that smoothly gasifies glucose and glucose-lignin mixture solution at 673 K, 25.7 MPa. The reactor system consists of three continuous reactors, which are a pyrolysis reactor, an oxidation reactor, and a catalytic reactor. We have investigated the reactions that occurred in each reactor and evaluated favorable residence time. We have succeeded in achieving high gasification efficiency based on carbon up to 96% at 673 K, 25.7 MPa with total residence time of about 1 min. The main gaseous products were H-2 and CO2. The results indicated that this three-reactor system is promising for low-temperature (i.e., around 673 K) biomass gasification in supercritical water.