An antitumor agent containing carbon dioxide as an active ingredient of the invention can reduce in size or eliminate the tumor of a tumor patient who has difficulty in undergoing surgical therapy, chemotherapy, radiation therapy, or immunotherapy almost without causing side effects. The antitumor agent containing carbon dioxide as an active ingredient of the invention is useful because the agent enables (1) the reduction in size or elimination of a tumor, (2) the suppression of tumor metastasis, and (3) the reduction in size or elimination of a tumor at a site different from a site where carbon dioxide is transdermally absorbed by the transdermal absorption of carbon dioxide through the skin in the vicinity of the tumor or the skin at a distance from the tumor. When the antitumor agent containing carbon dioxide as an active ingredient of the invention is used in combination with surgical therapy, chemotherapy, radiation therapy, or immunotherapy, the effect can be potentiated or side effects can be reduced as compared with monotherapy or combined modality therapy thereof.
