used and the viscosity of the solution while repetition of the passage of the solution through a capillary in a certain condition the molecular weight of the polysaccharide gradually decreases approaching a minimum value from which no further depolymerization occurs the minimum molecular weight also depends on the value of the shear rate applied higher shear rate gives lower minimum molecular weight thus the value of the shear rate applied is a dominant factor for the present method no substantial depolymerization occurs at too low shear rate generally a shear rate at least higher than 1 u00d7 10 4 sec 1 is necessary for the present depolymerization method in order to give such a high shear rate the pressure applied to the polysaccharide solution and the cross sectional area of the capillary are 20 800 kg cm 2 and 1 u00d7 10 4 100 mm 2 respectively but these values are not limited the present invention includes the finding that 1 increase in the concentration of the polysaccharide solution increases the efficiency of the depolymerization 2 addition of a solvent solvent b which is miscible with the solvent solvent a of the polysaccharide solution and does not dissolve the polysaccharide to the polysaccharide solution also increases the efficiency of the depolymerization the effect of the increase in the concentration of the polysaccharide solution reveals significantly when it is higher than 0 1 wt although the concentration is desirable to be as high as possible a part of the polysaccharide tends to.