Disclosed is a novel organosilicon-containing derivative of pullulan of which an organopolysiloxane moiety having a silethylene linkage is bonded to the glucose residue of pullulan through a urethane linkage. Different from conventional organopolysiloxane-modified pullulans, the inventive derivative is stable against attack of water and exhibits unique properties as a combination of the properties inherent in pullulan and in silicones. The organosilicon-containing pullulan of the invention can be easily prepared under mild reaction conditions by reacting an isocyanato group-containing organopolysiloxane with the glucosic hydroxy groups of pullulan.