PURPOSE: To provide a landing type or floating type of oil storage tank in water wherein oils are perfectly stored without generating marine pollution.

CONSTITUTION: For example, if an oil storage tank 1 is plunged under water 2, and is landed on the sea-bottom 3, the tank 1 is formed in a double shell structure comprising an inner steel shell 4 and an outer concrete shell 5, and a spacer 6 working also as a reinforcement between the said shells 4, 5. The space between the said shells is used as a ballast tank 7. If an oil storage space 8 in the inner shell 4 is fully loaded with oil, the ballast tank 7 is made empty to decrease the weight of the oil storage tank 1. Also, at least one small section 9 is provided in the oil storage space 8. If no oils are present in the oil storage space 8, ballast water is injected into the small section 9 so as to decrease the buoyancy of the oil storage tank 1 to an extent required for staying the tank on the sea-bottom.