A high-rate sedimentation tank includes a hopper configured to be supplied with raw water including floc, at least one circular orifice pipe disposed at a lower portion of the hopper and configured to have the floc deposited therein as a sludge while passing the floc included in the raw water therethrough, and a sludge outlet configured to discharge the sludge deposited by passing through the circular orifice pipe to an outside of the hopper.