The present invention relates to an agent for potentiating an antitumor activity of a chemotherapeutic agent against a cancer containing cancer stem cells, which contains at least one kind of branched chain amino acid selected from isoleucine, leucine and valine, or a salt thereof. In addition, the present invention relates to an agent for treating a cancer containing cancer stem cells, which contains a combination of a chemotherapeutic agent and at least one kind of branched chain amino acid selected from isoleucine, leucine and valine, or a salt thereof. Furthermore, the present invention relates to an agent for preventing metastasis or recurrence of a cancer containing cancer stem cells, which contains a combination of a chemotherapeutic agent and at least one kind of branched chain amino acid selected from isoleucine, leucine and valine, or a salt thereof.
