An apparatus for producing a mixed solution, comprising a mixing vessel for preparing an aqueous mixed solution containing a dicarboxylic acid and a Nb compound and a filter for the aqueous mixed solution connected to the mixing vessel via a pipe, the mixing vessel being anticorrosive and equipped with a stirring unit, a heating unit and a cooling unit for the aqueous mixed solution, wherein the aqueous mixed solution prepared in the mixing vessel is fed to the filter via the pipe and filtered in the filter under increased pressure.