The inventive concept provides a substrate treating apparatus. The substrate treating apparatus includes a chamber having an inner space therein; a fluid supply unit having a supply line configured to supply a treating fluid to the inner space and a fluid supply source configured to supply the treating fluid to the supply line; a first exhaust unit configured to exhaust the inner space; a second exhaust unit configured to exhaust the supply line; and a controller configured to control the fluid supply unit, the first exhaust unit, and the second exhaust unit, and wherein the controller controls the fluid supply unit and the second exhaust unit so a pressure of the supply line is maintained at a critical pressure of the treating fluid or above during at least a part of a standby step for keeping a substrate outside the inner space before introducing thereof into the inner space.