A liquid cyclone is configured for inflowing raw water containing impurities as targets of collection to be forced to swirl inside to spin down impurities contained in raw water, an inflow pipe is connected with an upper portion of the liquid cyclone to supply the liquid cyclone with raw water, and configured for supplied raw water to be forced to swirl inside the liquid cyclone, a connecting portion is connected with a lower portion of the liquid cyclone, and configured with a discharge port to discharge spun down impurities from the liquid cyclone, an impurity collector is connected to the liquid cyclone with the connecting portion in between, and configured to collect impurities discharged from the liquid cyclone, an obstacle is disposed in or near the discharge port, , and configured to prevent impurities collected in the impurity collector from backing up into the liquid cyclone, and an outflow pipe is connected with a top portion of the liquid cyclone, and configured for raw water having got rid of impurities to outflow as treated water from the liquid cyclone, whereby impurities separated from raw water is prevented from being re-mixed in raw water, allowing for an enhanced separation performance.