Enzymes or microorganisms are immobilized by bringing a first aqueous solution into contact with a second aqueous solution containing metal ions having a valence of 3 or more. The first solution contains enzymes or microorganisms, and at least one immobilizing agent selected from the group consisting of Xanthan gum and derivatives thereof. The immobilizing agent is thereby hardened in a state to enclose the enzymes or microorganisms. Preferably, the metal ions are iron, tin, manganese or titanium ions.