In the hydrogenation purification of crude terephthalic acid, at the stage of mixing crude terephthalic acid with water into a slurry, conducting heating/dissolving operation to thereby convert the same to an aqueous solution and subjecting the aqueous solution to a hydrogenation purification reaction operation, the variation of concentration of prepared/supplied slurry has often caused disordering of the continuous operation of the purification reaction. Thus, it has been demanded to prepare and supply a slurry of stable concentration wherein the range of variation of slurry concentration is reduced. The amount of crude terephthalic acid supplied is regulated at given ratio (given concentration) on the basis of the amount of water fed into a slurry preparation tank with agitator, and mixing thereof is conducted to thereby prepare a slurry (first regulation). While conducting external circulation of the slurry by means of a pump, the concentration thereof is measured, and the amount of water fed from the side of pump suction is regulated into a given concentration so as to attain regulation of the slurry concentration (second regulation). Thus, a slurry with narrowed range of concentration variation is obtained. Thereafter, in this process, by means of a high-pressure pump branched from circulation line, the prepared slurry is continuously supplied to heating/dissolving operation.