NOVELTY - The method comprises subjecting biomass to extractive treatment in which the biomass is heated in aqueous phase at a pressure in excess of 5 bar and at 160 degrees C to separate polysaccharides from the biomass. The pH of the biomass subjected to the extractive treatment is not actively lowered during the extractive treatment. After the treatment, the pressure is reduced in a controlled fashion so as not to decompose the fibrous structure of the biomass. The separated polysaccharides are recovered as a first fraction. The fibrous structure of the biomass is recovered as a second fraction.    USE - The method is useful for separating fibrous structure and polysaccharides from wood-based biomass such as peat moss, peat, chips or sawdust of hardwood/softwood, where the polysaccharide is used as a fuel e.g. biodiesel or as a raw-material for the production of chemical products in the chemical, foodstuff and pharmaceutical industry, and where the fibrous matrix is used for the production of fibrous products such as paper or cardboard (all claimed).    ADVANTAGE - The method is capable of simply, efficiently and economically separating the polysaccharides from the biomass in large-scale without disintegrating valuable compounds and any addition of chemicals, and produces high quality fibrous products with reduced chemical consumption.    DETAILED DESCRIPTION - The method comprises subjecting biomass to extractive treatment in which the biomass is heated in aqueous phase at a pressure in excess of 5 bar and at 160 degrees C to separate polysaccharides from the biomass. The pH of the biomass subjected to the extractive treatment is not actively lowered during the extractive treatment. After the treatment, the pressure is reduced in a controlled fashion so as not to decompose the fibrous structure of the biomass. The separated polysaccharides are recovered as a first fraction. The fibrous structure of the biomass is recovered as a second fraction. The first and second fractions are subjected to further treatment separately from each other, and then combined with mechanical pulp. The polysaccharides are separated essentially in the form of oligomers and polymers. The polysaccharides are hydrolyzed to monomers to form a hydrolysate. The polysaccharide fraction is separated from the biomass and then fuel pellets are produced from the lignocellulosic material formed by the fibrous matrix or the polysaccharide fraction. The polysaccharide-based fraction is separated before cellulose cooking to facilitate impregnation of cooking chemicals and to reduce consumption of chemicals and energy used in the cooking and then the fraction is recycled to and incorporated into the mass. The water is kept in aqueous phase during the whole extractive treatment to prevent decomposition of the fibrous structure. The temperature of the extraction is 190-240 degrees C. The pressure of the extraction is 0.5-10 MPa. The extraction is carried out continuously or batch-wise. The 30-95 wt.% of the hemicelluloses of the fibrous structure are removed by extraction. The pH of the extraction solution is 4.5-3.5. The additives such as heat-resistant enzymes working as catalysts, are incorporated into the mass before extraction, and modify the properties of the mass such as adjuvants promoting extraction.    DESCRIPTION OF DRAWING(S) - The diagram shows carbohydrate compositions of various Sphagnum species.