To provide a preparation method for efficiently preparing a large volume of cold seawater for maintaining the freshness of fish and shellfish and an apparatus for the same.

The seawater from which solids, such as sand, are removed by a filter device 2, is introduced into an ozone treating vessel 6 where the microorganisms included in the seawater are sterilized by the ozone injected therein and thereafter the organic matter of the dead microorganisms or the like is made into a frothy form by air bubbles within a separation vessel 8 and is removed to the outside of the system. The residual oxidant included in the seawater is removed within an activated carbon filter vessel 11. When a specified volume of the seawater subjected to filtration and sterilization accumulates in a water storage tank 15, the seawater is automatically sent to a cold water storage tank 17. The seawater is cooled to a prescribed temperature in a cooling tank 17a of an upper part and is stored in a cold seawater storage tank 17b of a lower part and is supplied to the outside of the system at need.