Abstract: Polymer dispersing agent comprising a graft polymer prepared by graft-polymerizing an organopolysiloxane having a mercapto group on the side chain with at least one acrylic or methacrylic monomer, and epoxy resin composition comprising epoxy resin, silicone resin and the above polymer dispersing agent. The polymer dispersing agent has an excellent effect for dispersing the silicone resin into the epoxy resin, and thereby the epoxy resin composition thus prepared has high dispersion stability and exhibits improved impact resistance and relaxation of residual internal stress and is useful as a sealing agent for IC, a coating agent, a paint composition, an adhesive, and a prepreg.