It is intended to provide a cancer metastasis inhibitor which shows a potent effect of inhibiting cancer metastasis comparable to bikunin, has no side effect and can be obtained in a large amount at a low cost. As the results of studies on the cancer metastasis inhibitory effect by administration of soybean Kunitz trypsin inhibitor (KTI), a potent activity of inhibiting cancer metastasis was found out. Since KTI is contained in soybean whey, which is yielded in a large amount in preparing isolated soybean protein and the like from soybeans, and soybean whey can be obtained at a low cost, KTI extracted and purified in a large amount from soybean whey is clinically applicable.