A substrate processing apparatus includes: a temperature raising part for raising a temperature of a first sulfuric acid; a mixing part for mixing the first sulfuric acid where the temperature is raised by the temperature raising part with a moisture-containing liquid to generate a mixed solution; and a discharging part for discharging the mixed solution onto a substrate inside a substrate processing part. The mixing part includes: a joining portion where a sulfuric acid supply line through which the first sulfuric acid where the temperature is raised by the temperature raising part flows and a liquid supply line through which the first sulfuric acid where the temperature is raised by the temperature raising part and the moisture-containing liquid flows are joined; and a reaction suppression mechanism for suppressing a reaction between the first sulfuric acid and the moisture-containing liquid in the joining portion.