To realize all the long time unrealized idea of loading fresh water existing in Japan on a crude oil tanker on an occasion of her voyage to the Middle East, a practical and economical method in which a huge amount of fresh water in good quality can be loaded is provided, the method solving the difficulty of washing the tanker's tank, the difficulty have been preventing the transportation of good quality water, and solving the problem of using the tanker's ballast water, the usage having been unpractical due to a small loading amount.

A crude oil tank is provided inside a ship's hold that is for loading fresh water. The tank can be moved along guides vertically. The loading space for fresh water can be widened as the tank moves upward by the buoyant force generated when loading the fresh water and/or by an external power and the like.