Fluid supply unit and substrate treating apparatus having the same

An apparatus for treating a substrate includes a process chamber having a treatment space for treating the substrate; a fluid supply unit for supplying fluid to the treatment space, wherein the fluid supply unit includes: a supply pipe connected to the treatment space to supply the fluid to the treatment space; a pump installed in the supply pipe to provide flow pressure to the fluid; a vent line installed between the pump and the process chamber to discharge pressure in the fluid to an outside; a relief valve installed in the vent line to open and close the vent line; and a reservoir installed between the pump and a branch point wherein the vent line branches from the supply pipe, wherein the reservoir has a cross-sectional area larger than a cross-sectional area of a portion of the supply pipe located between the pump and the branch point.