An antitumor agent comprising carbon dioxide as an active ingredient of the present invention can reduce the tumor volume or eradicate the tumor of a patient who is difficult to take a surgical therapy, a chemotherapy, a radiation therapy or an immunotherapy. The antitumor agent comprising carbon dioxide as an active ingredient of the present invention is useful because the agent enables (1) the reduction in size or elimination of a tumor, (2) the suppression of tumor metastasis and also (3) the reduction in volume or elimination of a tumor at a part different from the part where carbon dioxide is transdermally absorbed by the transdermal absorption of carbon dioxide through the skin near the tumor or the skin far from the tumor. When the antitumor agent comprising carbon dioxide as an active ingredient of the present invention is used in combination with a surgical therapy, a chemotherapy, a radiation therapy or an immunotherapy, the effect can be potentiated or side effects can be reduced as compared with a monotherapy or a multidisciplinary therapy thereof
