Provided is an etchant composition for a multilayered metal film comprising both a layer comprising copper and a layer comprising molybdenum, the etchant composition: being capable of etching en bloc a multilayered metal film comprising a layer constituted of copper or an alloy including copper as the main component and a layer constituted of molybdenum or an alloy including molybdenum as the main component; being effective in preventing the molybdenum layer from being undercut; making it easy to regulate the component concentrations so as to accommodate the cross-sectional shape control and cross-section; and being stable. Also provided are a method of etching using the etchant composition and a method for prolonging the life of the etchant composition. The etchant composition according to the present invention is an etchant composition for use in etching en bloc a multilayered metal film comprising a layer constituted of copper or an alloy including copper as the main component and a layer constituted of molybdenum or an alloy including molybdenum as the main component, and comprises hydrogen peroxide, an organic acid, an amine compound, an azole, and a hydrogen peroxide stabilizer (no inorganic acid is contained therein).