a spin chuck for holding a substrate thereon by suction, for spinning the substrate, and for moving up and down the substrate;

an upper bowl and a lower bowl surrounding the spin chuck for receiving a processing solution by which the surface of the substrate is processed, the upper bowl having an entrance;

an air outlet under the lower bowl for exhausting air by negative air pressure from the upper and lower bowls; and

a flow separation protrusion formed within the upper bowl, the flow separation protrusion preventing reflow of an air flow around the substrate by separating the air flow into an upward air flow and a downward air flow, then directing the upward air flow out the upper bowl through the entrance, and directing the downward air flow from the upper bowl through the air outlet,

wherein the flow separation protrusion is shaped into a ring around an inner wall of the upper bowl, and a section of the flow separation protrusion is so configured that a surface of the flow separation protrusion facing the substrate is inclined at a predetermined angle, an upper surface of the flow separation protrusion is less inclined than a lower surface thereof, and the flow separation protrusion is triangle-shaped having a vertex facing an outer-diameter edge of the substrate.