The substrate processing apparatus and the substrate processing method of the present invention generate an etching solution having a nitrite concentration suitable for etching by efficiently generating nitrous acid and improve the etching efficiency. The substrate processing apparatus (10) processes the semiconductor wafer (W) using an etchant (L) containing hydrofluoric acid and nitric acid, has a storage tank (210) for storing an etching solution (L), a concentration sensor (256) for measuring the concentration of nitrous acid in the etching solution (L), an alcohol supply unit (310) that supplies IPA to the etching solution (L) to maintain the concentration of nitrous acid at a predetermined value or more, and a substrate processing unit (100) that supplies the etching liquid (L) in the storage tank (210) to the semiconductor wafer (W).