In this study, an effective method for purifying of crude agar was attempted, and at the same time, the effect of crude protein and ash contained in impurified agar on the gel strength of the agar were investigated. In order to reduce the content of protein of crude agar, the agar extract was treated with a protein precipitant such as tricholoroacetic acid(TCA) or perchloric acid(PCA), whereas washing with deionized water was applied to decrease the ash content of agar extract. Among the protein precipitants used in the experiment PCA reduced the crude proteins of crude agar most efficiently; addition of 0.01% PCA resulted in the reduction of crude protein content by 3%, and the gel strength of agar thereby increased from 220g/$cm^{2}$ to 402g/$cm^{2}$. High ash content of crude agar was removed by means of washing treatment and it decreased from 8.1% to 2.7%, leading to the gel strength of 530g/$cm^{2}4$.