PURPOSE: To industrially and advantageously obtain high-purity crystals of benzoic acid by suspending and washing crude crystals of the benzoic acid containing phthalic acid or phthalic anhydride as an impurity in warm water under specific conditions, and efficiently separating and removing the aforementioned impurity according to a simple operation.

CONSTITUTION: For example, toluene is oxidized with molecular oxygen in the presence of a heavy metal catalyst such as cobalt in the liquid phase and benzoic acid is distilled and separated to provide crude crystals of benzoic acid containing phthalic acid or phthalic anhydride in an amount of, e.g. 300-3,000ppm, especially 500-2,000ppm (expressed in terms of the phthalic acid) as an impurity. The resultant crystals are then suspended and washed in warm water at 60-90°C, preferably at 70-85°C for ≥0.5hr, preferably ≥1hr to efficiently separate and remove the above-mentioned impurity and simply provide the objective high-purity crystals of the benzoic acid with ≤50ppm content of the above-mentioned impurity on an industrial scale. Furthermore, if phenols are contained as the impurity in the benzoic acid, the phenols can also be purified together.