a polyamic acid ester represented by the formula str6 where r 1 and r 2 are each the same or different group having carbon atoms of one or more ar 1 is a bivalent aromatic group and ar 2 is a tetravalent aromatic group the polyamic acid ester varnish used in the present invention can be obtained by homopolymerizing an aminodicarboxylic acid ester to obtain a polyamic acid or reacting an aromatic diamine with a tetracarboxylic acid derivative to obtain a polyamic acid and then heating the resulting polyamic acid in an organic solvent containing alcohol to esterify the polyamic acid or reacting the polyamic acid with a dehydrating agent the tetra carboxylic acids and their derivative includes ester acid anhydride and acid chloride acid chloride is preferred from the point of preparation the reaction for preparation of polyamic acids is generally carried out at 20 u00b0 200 u00b0 c in solvents such as n methylpyrrolidone nmp dimethylformamide dmf dimethylacetamide dmac dimethylsulfoxide dmso dimethyl sulfate sulforan butyl lactone cresol phenol halogenated phenol cyclohexanone dioxane tetrahydrofuran and acetophenone examples of the aminodicarboxylic acids and their derivatives used in the present invention includes 4 aminophthalic acid 4 amino 5 methylphthalic acid 4 p anilino phthalic acid 4 3 5 dimethyl 4 anilino phthalic acid and esters acid anhydrides and acid chlorides thereof examples of the aromatic diamines used in the present invention to provide the vibalent aromatic group ar 1 are p phenylenediamine p pda 2 5 diaminotoluene 2 5 diaminoxylene diaminodurene 2 3 5 6 tetramethylphenylenediamine 2 5 diamino benzotrifluoride 2 5 diaminoanisole 2 5 diaminoaceto phenone 2 5 diaminobenzophenone 2 5 diaminodiphenyl 2 5 diaminofluorobenzene benzidine o tolidine o tld m tolidine 3 3 39 5.