The inventive concept provides an apparatus for treating a substrate. The apparatus comprises a processing container having an inner space; a support unit configured to support and rotate the substrate in the inner space; a liquid supply unit configured to supply a treating liquid to the substrate supported by the support unit; and an exhaust unit configured to exhaust an air flow from the inner space, wherein the exhaust unit includes an air flow guide duct with an inlet provided to introduce the air flow into the air flow guide duct in a tangential direction to a rotating direction of the substrate supported on the support unit.