Abstract: An object of the present invention is to provide an antitumor aqueous solution and an anticancer agent, both of which can kill cancer cells while having virtually no effects on normal cells, and to provide methods for producing the antitumor aqueous solution and the anticancer agent. The method of the invention for producing an antitumor aqueous solution for killing cancer cells includes an aqueous solution preparation step of preparing an aqueous solution through addition, to water, of a solute containing at least one of disodium hydrogen phosphate (Na2HPO4), sodium hydrogen carbonate (NaHCO3), L-glutamine, L-histidine, and L-tyrosine disodium dihydrate (L-tyrosine.2Na.2H2O); and a plasma irradiation step of irradiating the aqueous solution with atmospheric pressure plasma generated in a plasma generation region by means of a plasma generator.