An apparatus for treating a substrate is provided. The apparatus for treating the substrate includes a high pressure chamber to provide a treatment space to perform a process of treating the substrate using a process fluid, a fluid supply source to provide the process fluid to the high pressure chamber, a fluid supply unit to supply the process fluid to the treatment space of the high pressure chamber, an exhaust unit to exhaust the process fluid in the high pressure chamber, and a pre-vent unit to vent a process fluid remaining inside a supply line.