To reduce vibration in a vertical direction without sacrificing a righting performance and stability of the floating body by providing a sealed air chamber reducing vibration in the vertical direction from a spring effect of compressibility of air in a liquid tank side of the floating body.

The sealed air chamber 14 is provided on a side part of the liquid tank 11 having the floating body 12 of a structure, a precision instrument or a device or the like. The sealed air chamber 14 and the liquid tank 11 are communicated at a lower part and liquid 13a flowing in and out the sealed air chamber 14 is always pressurized at higher pressure than air pressure. Vibration of the floating body 12 in the vertical direction is reduced by a spring effect from compressibility of air in a liquid tank side part.