NOVELTY - Pretreating a lignocellulosic biomass comprises: (a) hydrolyzing biomass with an acid solution to obtain a liquid fraction-1 (7) containing hemicellulose, and a solid fraction-1 (6) containing cellulose; (b) separating the liquid fraction-1 and the solid fraction-1; (c) drying (8) the solid fraction-1; (d) boiling (10) the dried solid fraction-1 in the presence or absence of an organic solvent to obtain a solid fraction-2 and a liquid fraction-2; (e) separating the solid fraction-2 and the liquid fraction-2; and (f) optionally treating; and (g) enzymatically hydrolyzing the solid fraction-2.    USE - The process is useful for pretreating a lignocellulosic biomass. The liquid fraction-1 is used in biotechnological methods of sugar conversion comprising ethanol fermentation of 6C-sugar by yeast, solvent fermentation by a bacterium, ethanol fermentation of 5-6C-sugar by yeast, bacterium or a genetically modified yeast or production of filamentous fungi (all claimed), where the lignocellulosic biomass is useful for producing biomass fuel, which is useful for agricultural waste such as wood, straw, or plantations such as Miscanthus.    ADVANTAGE - The method is capable of easily pretreating the lignocellulosic biomass to cellulose and allowing its transformation, recovers sugars with high output, and chemically, enzymatically, microbiologically treats cellulose to improve enzymatic hydrolysis.    DETAILED DESCRIPTION - Pretreating a lignocellulosic biomass comprises: (a) hydrolyzing biomass with an acid solution to obtain a liquid fraction-1 (7) containing hemicellulose in the form of hydrolysis products and acid, and a solid fraction-1 (6) containing cellulose and lignin; (b) separating the liquid fraction-1 and the solid fraction-1; (c) drying (8) the solid fraction-1; (d) boiling (10) the dried solid fraction-1 in the presence or absence of an organic solvent in a medium comprising at least one inorganic hydrated salt having a metal compound of formula (MXn.n1H2O) to obtain a solid fraction-2 and a liquid fraction-2 containing the hydrated inorganic salt; (e) separating the solid fraction-2 and the liquid fraction-2; (f) optionally treating the solid fraction-2; and (g) enzymatically hydrolyzing the solid fraction-2, where the liquid fraction-1 is used for growth of the microorganism producing enzymes necessary for the enzymatic hydrolysis.    M = metal comprising 1-13 groups of the periodic table;    X = anion;    n = 1-6; and    n1 = 0.5-12.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a method.    Reactor (2)    Solid fraction-1 (6)    Liquid fraction-1 (7)    Drying solid fraction-1 (8)    Boiling solid fraction-1. (10)