Disclosed herein is a semi-submersible offshore structure having storage tanks for liquefied gas, which is constructed so as to improve workability in marine offloading of the liquefied gas stored in the storage tanks while reducing an influence of sloshing. The offshore structure is anchored at sea and has liquefied gas. The offshore structure includes a storage tank storing liquefied gas, a plurality of columns partially submerged under the sea level and each having the storage tank therein, and an upper deck located on the plurality of columns to connect the columns to each other.