PURPOSE: To obtain a polyamic acid having high polymerization degree even when an organic polyamine having electron-attractive substituent on an aromatic nucleus is used as an organic amine component, by carrying out polymerization reaction in a solvent containing a carboxylic acid.

CONSTITUTION: A carboxylic acid (preferably ≤5C carboxylic acid such as formic acid, etc.) is blend with a solvent such as N,N-dimethyl formamide, etc., preferably in a ratio of 30W50wt%. Then, in the solvent containing the carboxylic acid, (A) an aromatic compound (e.g. 4,4-diaminodiphenylsulfon, etc.) having an electron-attractive substituent on an aromatic nucleus and ≥2 amino groups is reacted with (B) a compound (e.g. 1,2,4,5-benzene tetracarboxylic dianhydride, etc.) having ≥2 acid anhydride groups, preferably at 5W50°C to provide the aimed polyamic acid.