In a method of producing pyromellitic acid by liquid-phase oxidizing 2,4,5-trimethylbenzaldehyde in a water solvent with molecular oxygen, a recrystallization mother liquor separated in a recrystallization step is recycled to the oxidation step after a part of the water solvent is removed. With this method, the loss of pyromellitic acid and the oxidation catalyst can be minimized and the burden of discharging the waste water is reduced without lowering the efficiency of the liquid-phase oxidation reaction.