An object of the present invention is to provide a method for preparing a sulfuric acid base copper electrolytic solution used for formation of an electro-deposited copper film comprising a surface excellent in smoothness and gloss when formed by using the solution just after preparation and is prepared by using mono-sulfides. To achieve the object, a sulfuric acid base copper electrolytic solution is made to contain a sulfonated active sulfur compound, the bis(3-sulfopropyl)disulfide which is recommended for formation of a glossy electro-deposited copper film. And the bis(3-sulfopropyl)disulfide contained is obtained by converting a 3-mercapto-1-propanesulfonic acid into the bis(3-sulfopropyl)disulfide in an aqueous solution of the 3-mercapto-1-propanesulfonic acid by an oxidation reaction. In the oxidation reaction, an air bubbling method is preferably used to prevent oxidative decomposition of the 3-mercapto-1-propanesulfonic acid.