Anticancer agent and side-effect-alleviating agent

The purpose of the present invention is to provide an anticancer agent for potentiating an antitumor effect, alleviating side effects, and further extending the survival rate by concomitant use with a component having an anticancer effect. An anticancer agent combining arctigenin and a component other than arctigenin that has an anticancer effect, in which the anticancer agent may be a combination drug or may be a kit configured from a formulation containing arctigenin and a formulation containing a component that has an anticancer effect, and the concomitant use of arctigenin and the component having an anticancer effect more strongly inhibits tumor growth and reduces the proportion of cancer stem cells in the tumor, making it possible to extend the total survival time and to alleviate side effects caused by the component having an anticancer effect.