A process for preparing an immobilized microorganism which comprises cultivating a microorganism in a culture broth, treating the broth with glutaraldehyde when cultivation being completed, collecting microbial cells from the broth, admixing the microbial cells with an aqueous solution of a polysaccharide having 10 w/w % or more of sulfate moiety in the molecule thereof and then gelling the polysaccharide in the resulting mixture to entrap the microbial cells within the gel matrix of the polysaccharide. The process is suitable for the industrial preparation of immobilized microorganisms.