A polyamide-imide resin which can give a varnish having a high resin content, excellent in storage stability and heat resistant final products such as insulated wire can be produced by reacting an aromatic diisocyanate with a tricarboxylic acid anhydride in the presence of a basic solvent while adjusting the resin content at 40% by weight or more, and adding a compound having one active hydrogen atom before, during or after the above-mentioned reaction so as to adjust the reduced viscosity of the resin to 0.27 or lower.