NOVELTY - Hydrothermal decomposition system of biomass, comprises biomass supply portion for supplying biomass feedstock, hydrothermal decomposition portion for hydrothermally decomposing biomass feedstock, biomass solid content extraction portion for extracting biomass solid content from the hydrothermal decomposition portion, and slurry tank.    USE - The hydrothermal decomposition system is useful for producing sugar liquid and alcohol (all claimed).    ADVANTAGE - The method is cost-effective.    DETAILED DESCRIPTION - Hydrothermal decomposition system of biomass, comprises biomass supply portion for supplying biomass feedstock containing cellulose, hemicellulose, and lignin for pressurization under normal pressure, hydrothermal decomposition portion for hydrothermally decomposing biomass feedstock using pressurized hot water and dissolving lignin component and hemicellulose component in pressurized hot water, biomass solid content extraction portion for extracting biomass solid content from the hydrothermal decomposition portion, and slurry tank into which water is injected inside while connecting with biomass solid content extraction portion and into which extracted biomass solid content is injected and slurried. INDEPENDENT CLAIMS are included for the following:    (1) method for producing sugar liquid using the biomass feedstock, involves supplying biomass feedstock containing cellulose, hemicellulose and lignin under normal pressure or pressurization, hydrothermally decomposing the biomass feedstock by hydrothermal decomposition portion with pressurized hot water, dissolving the lignin component and hemicellulose component in the pressurized hot water, extracting biomass solid content from the hydrothermal decomposition portion, injecting the extract into slurry tank by connecting with the hydrothermal decomposition portion into which water is injected into inside, and is made into slurry-like biomass solid content, subsequently removing water from the slurry like biomass solid content and performing enzyme saccharification of biomass solid content by removing water to obtain sugar liquid; and    (2) method for producing alcohol, involves performing alcoholic fermentation using the sugar liquid obtained by the above-cited method and biomass feedstock.