According to the embodiment, a substrate treating device 10 for treating a semiconductor wafer W using an etchant L containing hydrofluoric acid and nitric acid includes a storage tank 210 that stores the etchant L; a concentration sensor 256 that measures a concentration of nitrous acid in the etchant L; an alcohol feeding line 280 that feeds IPA to the etchant L and maintains the concentration of nitrous acid to a predetermined value or more; and a substrate treating unit 100 that feeds the etchant L in the storage tank 210 to the semiconductor wafer W. The substrate treating device can improve the etching efficiency by efficiently generating nitrous acid, and thereby producing an etchant having a nitrous acid concentration suitable for etching.