A novel method and device for treating a liquid are provided, wherein a concentrated liquid recovered using a centrifugal separation method can be utilized as ballast water. The present disclosure relates to a method for treating a liquid that comprises a storage step including carrying out a physical treatment using a centrifugal force with respect to a supply liquid to be supplied to a storage means located inside a housing, wherein the method comprises carrying out an aquatic organism-inactivating treatment contained in a concentrated liquid obtained by the physical treatment using a centrifugal force and supplying to the storage means the concentrated liquid subjected to the aquatic organism-inactivating treatment.