To provide to a reagent composition used in quantitative measurement of lithium in aqueous solutions such as biological specimens and environmental liquid samples by a simple colorimeter or ultraviolet-visible light spectrophotometer immediately, a reagent composition which permits to measure a concentration of lithium by visual observation, and a method and apparatus for measuring lithium ion by using the reagent. A reagent composition for measuring lithium comprising F28 tetraphenylporphyrin compound as chelating agent, including further a water-soluble organic solvent, a pH modifier and a stabilizer, and a method and apparatus for measuring lithium ion by using the reagent.