Resins having a group reactive with an isocyanate group on a side chain are modified with isocyanatosiloxanes. In one form, the isocyanatosiloxane is a single end isocyanatosiloxane obtained by effecting addition reaction between a single end hydrogenosiloxane and an isocyanate group-containing organic silicon compound having one isocyanate group and at least one aliphatic unsaturated hydrocarbon group. A single end isocyanatosiloxane obtained by introducing an isocyanate group into a single end primary aminosiloxane is also useful. In another form, the isocyanatosiloxane is a polyfunctional isocyanatosiloxane which is obtained by effecting addition reaction between a hydrogenosiloxane having at least two SiH groups and an isocyanate group-containing organic silicon compound having one isocyanate group and one aliphatic unsaturated hydrocarbon group.