A liquid supply unit includes a tank having an inner space for storing the liquid, an inlet line for supplying the liquid from the liquid supply source to the inner space and having an inlet valve installed thereon, an outlet line for supplying the liquid from the tank to a nozzle or for recollecting the liquid to the tank and having an outlet valve installed thereon, a gas supply line for supplying a gas to the inner space and having a gas control valve installed thereon, an exhaust line for exhausting the inner space and having an exhaust valve installed thereon, a circulation line for circulating the liquid stored in the inner space, and a controller controlling the liquid supply unit so that the circulation line is pressurized while the liquid is supplied to the inner space.