Abstract: A substrate processing apparatus does not allow sudden boiling of supplementary liquid but rather sufficiently diffuses the supplementary liquid into processing liquid so that the temperature of the processing liquid does not decrease largely. De-ionized water (PW) is ejected through a plurality of de-ionized water blow-off openings (Ha) which are formed in a de-ionized water injection nozzle (10a). Inside an internal bath (1), which is located immediately below the de-ionized water blow-off openings, and close to a overflow part (20a) which is a top edge of a side surface (1a) of the internal bath (1), the de-ionized water reaches the surface of a phosphoric acid solution (PA). The de-ionized water thereafter reaches a gutter part (30a) of an external bath (2), as it is mixed with the phosphoric acid solution (PA), and flows into a reservoir part (30d) of the external bath (2).