To provide a reagent material for detecting the existence of copper ions in an aqueous solution, and a reusable reagent material for measuring the concentration of the copper ions.

The reagent material is acquired, by forming a titanium oxide sol by hydrolyzing a specified titanium oxide i.e. alkoxy titanium in a nitric acid aqueous solution under the condition of pH of 1 or lower, aging it for 100 hours or more, forming a membrane on a substrate and sintering it at a temperature of 100-600°C. The copper is deposited and attached to the reagent material, by irradiating the reagent material with ultraviolet rays. The copper ions are detected by the coloring of the copper. The concentration of the copper ions is quantified by the change in its color tone.