A substrate processing apparatus includes a mixing tank, a first opening/closing valve, a second opening/closing valve, a first flow rate measuring unit, a second flow rate measuring unit, a control unit, and a substrate processing unit. A first liquid and a second liquid are mixed such that the second liquid is mixed in an amount more than that of the first liquid. The first and second opening/closing valves open/close a first flow path and a second flow path, respectively. The first and second flow rate measuring units measure flow rates of the first and second liquids flowing through the first and second flow paths, respectively. The control unit controls opening/closing of the first opening/closing valve and the second opening/closing valve. The substrate processing unit processes a substrate by supplying a mixed liquid of the first and second liquids to the substrate.