The inventive concept provides a substrate treating apparatus. The substrate treating apparatus includes a process chamber that provides a treatment space, in which a substrate is treated, in an interior thereof, a fluid supply unit that supplies a fluid into the process chamber, and an exhaust units including an exhaust line, through which the fluid in the process chamber is exhausted, the fluid supply unit includes a supply tank, in which the fluid is stored, a supply line connecting the supply tank and the process chamber, a branch line branched from the supply line at a first point of the supply line, and a controller that controls the fluid supply unit, and the controller controls the fluid supply unit such that the fluid is drained from the supply line through the branch line shortly before the fluid is supplied into the process chamber.