A substrate processing apparatus includes: a holding unit that holds a substrate; a liquid discharge unit; a first supply unit; a second supply unit; and a control unit that controls each unit. The liquid discharge unit discharges a processing liquid to the substrate held by the holding unit. The first supply unit supplies the processing liquid to the liquid discharge unit. The second supply unit supplies steam to the liquid discharge unit. The second supply unit includes: a steam generator that generates steam; a supply line; a stabilizing mechanism; a pressure gauge that measures a pressure of the steam flowing through the supply line; and a pressure adjustment mechanism. The control unit controls the pressure adjustment mechanism so that the pressure of the steam measured by the pressure gauge becomes a preset pressure.