A substrate processing apparatus includes: a mixer that mixes a first phosphoric acid and an additive serving as raw materials of a processing liquid with each other at a predetermined mixing ratio, thereby preparing a mixed liquid; a mixing ratio corrector that corrects the mixing ratio of the raw materials of the processing liquid; a processing unit that processes a substrate with the processing liquid. The mixer includes a mixing tank that stores the mixed liquid, a first phosphoric acid supply that supplies the first phosphoric acid to the mixing tank, and an additive supply that supplies the additive to the mixing tank. The mixing ratio corrector includes a liquid line through which the mixed liquid is delivered from the mixer to the processing unit, and a second phosphoric acid supply connected to the liquid line so as to supply second phosphoric acid to the liquid line.