An apparatus for treating a substrate, the apparatus comprising: a processing container having an inner space; a support unit having a support plate configured to support and rotate the substrate in the inner space; a liquid supply unit supplying treating liquid to the substrate supported by the support unit; and an exhaust unit exhausting an air flow in the inner space, wherein the processing container includes a bottom wall and a side wall extending from the outside end of the bottom wall, the processing container including a first gas-liquid separator provided at the side wall.