PROBLEM TO BE SOLVED: To provide an antitumor agent which can reduce, eliminate, or suppress metastasis of a tumor of a tumor patient who has difficulty in undergoing a surgical therapy, chemotherapy, radiation therapy, or immunotherapy almost without causing side effects.SOLUTION: The present invention provides an antitumor agent containing carbon dioxide as an active ingredient and enabling (1) a reduction in size or elimination of a tumor, (2) suppression of tumor metastasis, and (3) a reduction in size or elimination of a tumor present at a site different from a site where carbon dioxide is transdermally absorbed, by 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 of the invention is used in combination with a surgical therapy, chemotherapy, radiation therapy, or immunotherapy, the effects can be potentiated or side effects can be reduced as compared with when these therapies are applied as a monotherapy or combined modality therapy.【課題】外科療法、化学療法、放射線療法、免疫療法が困難な腫瘍患者の腫瘍を副作用をほとんど伴わずに縮小もしくは消失させ、転移を抑制することができる抗腫瘍剤の提供。【解決手段】二酸化炭素を有効成分とする抗腫瘍剤。本発明の抗腫瘍剤は、腫瘍近傍皮膚あるいは腫瘍から離れた皮膚からの二酸化炭素の経皮吸収によって、（１）腫瘍の縮小もしくは消失、（２）腫瘍の転移の抑制、（３）二酸化炭素の経皮吸収部位とは異なる部位の腫瘍を縮小もしくは消失できる。また、本発明抗腫瘍剤を、外科療法、化学療法、放射線療法、免疫療法に併用すると、それらの単独療法もしくは集学的療法に比べて、効果を増強したり、副作用を低減できたりする。【選択図】なし
