Method for treating ballast water and device for treating ballast water used therefor

Provided are a novel method and device for treating a liquid that can be utilized for treating ballast water used in ships, etc. The method and device for treating a liquid provided herein allow aquatic organisms contained in a liquid to be sufficiently inactivated or separated by being configured in such a manner as to, in supplying the liquid containing aquatic organisms, carry out at least one of an aquatic organism-inactivating treatment and a physical treatment using a centrifugal force and then store the liquid in a storage means, and in discharging the liquid thus stored, determine whether or not the aquatic organism-inactivating treatment is required, carry out the treatment based on the determination, and carry out the physical treatment using a centrifugal force.