To set low the vertical natural frequency of a tank body and bring out a aseismatic structure function in the vertical direction as well, by installing the tank body on a floating body on a liquid and connecting the liquid below a plurality of tank bodies with connecting pipes.

When there are two tank bodies 1, one 1a of them is installed on a floating body 2a on the liquid 3a and the other tank body 1b is installed on a floating body 2b on the liquid 3b. A connecting pipe 4 is filled with the liquid 3. The surface of the liquid 3a is covered with a water-tight seal device 5a like a rubber film and also the surface of the liquid 3b is covered with a water-tight seal device 5b. In this time, the vertical natural frequency of the tank body becomes equal to the natural frequency of the liquid 3 in the U-shaped tube chiefly constituted of the connecting pipe 4. In this way, the vertical natural frequency of the tank bodies 1a, 1b can be set sufficiently low.