The present invention provides a process for preparing a self-curing resin having a blocked isocyanate group and a hydroxyl group in the molecule from a polymer 500 to 50000 in number average molecular weight and having a tertiary amino group and/or a quaternary ammonium group, and at least two isocyanate groups in the molecule, the process comprising the steps of partly blocking the isocyanate groups in the skeleton of the polymer, and reacting the remaining unblocked isocyanate groups with an alkanolamine to introduce hydroxyl groups into the polymer.