Provided are a varnish mainly containing a polyester imide and capable of forming a low-permittivity insulating layer, as well as an insulated electronic wire achieving low permittivity by using the varnish. The varnish mainly contains a polyester imide resin obtained by reacting a carboxylic acid including a dicarboxylic acid, or an anhydride or alkyl ester thereof (carboxylic acid or derivative thereof), an alcohol, and a diamine compound with one another. In the varnish, adjustment is made such that a molar ratio (OH/COOH) of hydroxyl groups of the alcohol to carboxyl groups of the carboxylic acid or derivative thereof becomes 1.9 or less or such that a content of highly polarized imide groups per unit amount of polyester imide chain is reduced by increasing molecular weights of the raw material monomers.