A substrate processing apparatus includes: a processing container, a mixing device, a liquid feeding path, and a controller. The processing container processes a substrate by immersing the substrate in a processing liquid. The mixing device mixes a phosphoric acid aqueous solution and an additive, to produce a mixed liquid to be used as a raw material of the processing liquid. The liquid feeding path feeds the mixed liquid from the mixing device to the processing container. The controller controls the substrate processing apparatus. The controller performs a control to feed the mixed liquid from the mixing device to the processing container in which the substrate is immersed, after a phosphoric acid concentration of the mixed liquid is regulated from a first concentration to a second concentration higher than the first concentration. The first concentration is a concentration when the phosphoric acid aqueous solution is supplied to the mixing device.