1,3-Naphthalenedicarboxylic acid is produced by oxidizing 1,3-dialkylnaphthalene in a liquid-phase with an oxygen-containing gas in the presence of a C2-C6 lower aliphatic carboxylic acid solvent and a catalyst comprising a heavy metal and a bromine compound. By regulating the ratio of the total number of bromine atoms fed into a reaction system to the total number of 1,3-dialkylnaphthalene molecules fed into the reaction system within a specific range, 1,3-naphthalenedicarboxylic acid is efficiently produced with low costs. Using 1,3-dimethylnaphthalene, as the starting 1,3-dialkylnaphthalene, which is produced by isomerizing dimethylnaphthalenes in a liquid phase in the presence of a catalyst comprising hydrogen fluoride and boron trifluoride together with a C5-C10 alicyclic saturated hydrocarbon having a five-membered or six-membered ring structure, a highly pure 1,3-naphthalenedicarboxylic acid is efficiently produced.