A problem of the invention is to provide a process of producing high purity terephthalic acid by selectively recovering p-toluic acid in wastewater which has hitherto been discarded and using it as a raw material of terephthalic acid.The invention is concerned with a process of producing high purity terephthalic acid, which is characterized in that a p-toluic acid recovery step of recovering p-toluic acid from a primary mother liquor and feeding it into an oxidation reaction step includes the following respective steps of: (I) an adsorption step of feeding, as a liquid to be treated, the primary mother liquor or a secondary mother liquor obtained by subjecting the primary mother liquor as cooled to solid-liquid separation into an adsorption tower filled with an adsorbing agent in which the breakthrough time of p-toluic acid is longer than the breakthrough time of benzoic acid to adsorb p-toluic acid and benzoic acid in the liquid to be treated to the adsorbing agent, (II) a feed stopping step of stopping the feed of the liquid to be treated into the adsorption tower at an arbitrary point of time of exceeding the breakthrough time of benzoic acid, (III) a desorption step of feeding a desorbing agent into the adsorption tower to desorb adsorbed p-toluic acid, and (IV) a circulation step of feeding p-toluic acid contained in the desorbing agent flown out from the adsorption tower into the oxidation reaction step.