An additive containing a hexafluorosilicic acid solution (H2SiF6+H2O) is sequentially inputted into a phosphoric acid solution pooled in an immersion bath from an additive input mechanism. Further, a trap agent containing a fluoroboric acid solution (HBF4+H2O) is inputted into the phosphoric acid solution from a trap agent input mechanism. F− which accelerates etching of a silicon nitride film is added as appropriate by sequentially inputting the additive and siloxane which increases by the sequential input is etched with hydrofluoric acid generated by decomposition of the fluoroboric acid, to thereby suppress a significant increase in the concentration of siloxane. This makes it possible to maintain respective initial etching rates of the silicon nitride film and a silicon oxide film.