It is to provide an agent for suppressing proliferation and/or inducing apoptosis of cancer cells that can be used for the treatment and chemoprophylaxis of cancer, wherein the agent is easily accessible and ingestible in daily dietary life and is prepared using a naturally-derived ingredient with reduced side effects, and a method for suppressing proliferation and/or inducing apoptosis of cancer cells in a subject in need thereof, and the like. By administering an agent for suppressing proliferation and/or inducing apoptosis of cancer cells comprising Sargassum horneri or a processed product thereof to a subject in need thereof, the proliferation of cancer cells can be inhibited and apoptosis of cancer cells can be induced in the subject.
