PURPOSE: To obtain the subject high-purity compound by reaction of a crude naphthalenedicarboxylic acid with a 6-20C aliphatic diamine.

CONSTITUTION: In producing this compound by reaction of a crude naphthalenedicarboxylic acid with a 6-20C aliphatic diamine, it is characteristic that the naphthalenedicarboxylic acid is reacted with virtually 0.9-3 molar times of the diamine at 50-200°C in the presence of such an amount of water as to be enough to be perfectly soluble for the naphthaenedicarboxylic acid diamine salt formed and the resultant reaction liquor is filtered at such a temperature as not to deposit the salt to remove insolubles and the resultant aqueous solution containing the salt is cooled to 40°C or lower to deposit the objective salt.