To completely prevent the generation of chlorine gas by previously dissolving a reducing agent in hydrochloric acid and repeating a process of alternately adding the equivalent quantity of the reducing agent and cobaltic acid lithium corresponding to the chemical reaction formula.

At the time of dissolving cobaltic acid lithium wasted in a lithium cell production process or in a waste lithium ion cell with hydrochloric acid, the reducing agent is previously dissolved in hydrochloric acid. As the reducing agent, oxalic acid or formic acid is used. Cobaltic acid lithium is dissolved in hydrochloric acid by dividedly and alternately adding the reducing agent and cobaltic acid lithium so that the reducing agent in hydrochloric acid is equivalent to cobaltic acid lithium corresponding to the chemical reaction formula. Because the reducing agent is previously dissolved in hydrochloric acid, a vigorous reaction in the reaction of cobaltic acid lithium with hydrochloric acid is suppressed and the generated chlorine gas forms hydrochloric acid by the reaction with the reducing agent.