To provide a method reducing the amount of new water (water afresh supplied to producing processes) used when producing terephthalic acid without deteriorating quality of the product.

This method comprises oxidizing p-xylene in a liquid reactional medium by molecular oxygen to obtain a slurry containing crude terephthalic acid, separating the crude terephthalic acid from the slurry as a solid, dissolving the crude terephthalic acid solid to produce a solution and carrying out hydrogenation treatment in the presence of a catalyst, carrying out solid-liquid separation of the obtained reactional product to obtain terephthalic acid and a reactional liquid and reducing at least a part of p-toluic acid and/or benzoic acid included in the obtained collected reactional liquid by photocatalytic reaction.