give an intermediate product next trimellitic anhydride was added to the intermediate product in an amount shown in each of table 1 to 3 and the mixture was reacted until a desired softening point was attained under the conditions of 210 u00b0 to 220 u00b0 c and 27 kpa to give a resin composition a 5 liter four neck flask equipped with a dehydration tube equipped with a rectifying tower a nitrogen inlet tube a stirrer and a thermocouple was charged with raw material monomers for a low molecular weight polyester shown in each of tables 1 to 3 and 10 parts by weight of dibutyltin oxide here a hot water at 95 u00b0 c was allowed to flow through the rectifying tower the mixture was reacted with heating from 185 u00b0 to 230 u00b0 c over a period of about 8 hours and further reacted under the reduced pressure of 52 kpa for 1 hour thereafter trimellitic anhydride in an amount shown in each of tables 1 to 3 was added thereto and the mixture was reacted until a desired softening point was attained under the conditions of 210 u00b0 to 220 u00b0 c and 27 kpa to give a resin composition in comparative.