An apparatus for processing a substrate includes a mixing nozzle having a mixing space that is shaped as an inverted cone, the mixing nozzle including a first inlet, a second inlet and an outlet installed at the mixing space; a first chemical supply unit connected to the first inlet; a second chemical supply unit connected to the second inlet; and a supply tank connected to the outlet, wherein the first chemical supply unit is configured to supply a first chemical solution to the mixing space through the first inlet, the second chemical supply unit is configured to supply a second chemical solution to the mixing space through the second inlet, and the supply tank is supplied with, through the outlet, a solution mixture that is formed of the first chemical solution and the second chemical solution after being mixed in the mixing space.