The purpose of the present invention is to provide a method for manufacturing a new disinfectant from sodium hypochlorite that has degraded in quality during storage. A method for manufacturing a novel disinfectant from a solution containing hypochlorite ions, chlorate ions, and chloride ions, wherein the method includes: a first reaction step for adding sulfuric acid to the solution and generating chlorine gas; a step in which, in a recovery liquid A, the generated chlorine gas is caused to react with sodium hydroxide or calcium hydroxide and recovered as hypochlorite ions; a second reaction step for adding, to a reaction mother liquid after the first reaction step, sulfuric acid having a higher concentration than that in the first reaction step, and generating chlorine dioxide gas; a step in which, in a recovery liquid B, the generated chlorine dioxide gas is caused to react with sodium hydroxide and hydrogen peroxide and recovered as chlorite ions; and a step for mixing the recovery liquid A and the recovery liquid B and obtaining a novel disinfectant.