The present invention provides an anticancer agent that is readily absorbed into cells but is not chemically altered and does not induce immunity when metabolized inside an organism and that can be used in combination with other anticancer agents. As the result of diligent research, the inventors of the present invention discovered de novo that ozone water than can be preserved for a long time has anticancer activity. It was also found that the effect of the ozone water was greatly increased when the ozone water was used in combination with existing anticancer agents.
