The chemopreventive effect of the polyphenols abundant in green tea on carcinogenesis has been attracting attention in recent years. Among tea polyphenols, epigallocatechin gallate (EGCG) has been studied as a preventive substance for carcinogenesis. We investigated the chemopreventive effect in a group treated with EGCG in vitro and in vivo using mouse mammary epithelial cells RIII/MG. In the in vitro experiment, crude catechin (catechin) containing 50% or more EGCG significantly inhibited the growth of RIII/MG cells, which were precancerous cultured cells. Many cells died, and a DNA ladder was observed. In the in vivo experiment, RIII/MG cells formed a tumor after 13 weeks in a group without catechin treatment, and the tumor formation rate in the 20th week was 40%. In a group treated with 0.1% catechin, a tumor began to grow in the 13th week, and the tumor formation rate in the 20th week was 20%. In a group treated with 1% catechin, no tumor was detected even in the 20th week. There was no significant difference in the change in body weight between the catechin treatment groups and the non-treatment group during the observation period. Tissue samples were stained by the nick-end-labeling method and apoptosis was observed in many cells. Based on the above findings, EGCG inhibited growth in the mouse viral mammary epithelial carcinogenesis model RIII/MG, and induced apoptosis, suggesting a clinical usefulness of EGCG as a chemopreventive substance.