The purpose of the present invention is to provide a carcinogenesis inhibitor which has no side effects and exhibits excellent effects when orally administered. The carcinogenesis inhibitor accordingto the present invention, which comprises a hydantoin derivative or a pharmaceutically acceptable salt thereof as an active ingredient, exhibits effects of inhibiting polyp formation and inhibiting cell proliferation and has a high safety without posing side effects. Thus, the carcinogenesis inhibitor is highly useful in medicines and foods such as supplements for preventing the occurrence and progress of cancer, inhibiting the recurrence and metastasis of cancer and, furthermore, exerting therapeutic effects.