A tumor can be regressed or eradicated with little adverse side effects using a liquid medicine according to the present invention, which comprises a liquid having carbon dioxide dissolved therein and is to be administered using a liquid injection means, or by a therapeutic method according to the present invention, which uses a liquid medicine to be administered using a liquid injection means and comprising a liquid having carbon dioxide dissolved therein. When the liquid medicine according to the present invention, which comprises a liquid having carbon dioxide dissolved therein and is to be administered using a liquid injection means, and the therapeutic method according to the present invention, which uses a liquid medicine to be administered using a liquid injection means and comprising a liquid having carbon dioxide dissolved therein, are used in combination with a surgical therapy, a chemotherapy, a radiation therapy or an immunotherapy for tumors, the effect can be potentiated or adverse side effects can be reduced compared with a case in which each of the therapies is carried out singly or a multimodal treatment of the therapies is carried out.
