Polysaccharide is wet pulverized by ejecting a liquid dispersion of polysaccharide from a pair of nozzles under a high pressure of 70 to 250 MPa to form a pair of jets so that the jets collide against each other. The nozzles are oriented to define a collision angle of 95-178.degree. and the number of pulverizing cycles is adjusted such that the polysaccharide is pulverized to an average particle length which is equal to or less than 1/4 of the initial average particle length while undergoing a reduction in degree of polymerization of less than 10%.