of terephthalic acid as the product is determined by content of 4 carboxybenzaldehyde and transmittance at 340 nm these control values are maintained within a fixed range when the treatment is carried out in a stationary manner when the control values have come outside the fixed range it is necessary that the catalyst in the reaction vessel be partly exchanged for a new one to increase the efficiency of the treatment after the partial exchange of catalyst and when the hydrogenation of crude terephthalic acid has been restarted the resultant purified terephthalic acid usually has a very small content of 4 carboxybenzaldehyde on account of high activity of catalyst and accordingly the resultant purified terephthalic acid has a high transmittance at 340 nm although depending upon the degree of deactivation of catalyst usually 20 70 of the deactivated catalyst in a reaction vessel is exchanged for a new one after the deactivated catalyst in the reaction vessel has been in part exchanged with a new one in a manner as above an aqueous slurry of crude terephthalic acid is fed into the reaction vessel and made into an aqueous solution under a high temperature and pressure to restart the treatment according to the invention crude terephthalic acid containing 4 carboxybenzaldehyde in a larger amount than that in.