Adsorption experiments for a mixed acid (phosphoric acid+ acetic acid) with a silica gel adsorbent and the adsorption-assisted batch crystallization of phosphoric acid were performed. In the adsorption experiments, the impurity concentration of acetic acid in the solution was efficiently reduced by the selective adsorption of acetic acid from the mixed acid by silica gel. The crystallization temperature and the weight fraction of silica gel adsorbent were varied as the operative variable for the adsorption-assisted crystallization. It was found that crystals of phosphoric acid having the purity more than 99% were obtained. As the weight fraction of adsorbent increased, the purity of phosphoric acid's crystals also was slightly increased. When the crystallization temperature was lowered, the yield of phosphoric acid's crystals was also increased, but the purity of phosphoric acid's crystals did not change much. As for the adsorption-assisted crystallization using silica gel adsorbent, the crystals of phosphoric acid changed from needle-like crystals to plate-like crystals as the weight fraction of the adsorbent increased. It was consequently suggested that the smaller crystals of phosphoric were generated in the solution, the higher purity of phosphoric acid's crystals were formed.