To provide an offshore structure and its construction method which can reduce influence on marine environment in the construction of offshore structure such as a heliport or the like and can easily remove or transfer after use and reduce the construction period and expences and contribute to regional economy.

Concrete blocks 2 as wide as approximate lengthwise and lengthwise dimensions of 50 m×50 m on a plane in the marine construction position, for instance, are arranged in a plane to form an offshore heliport or the like by use of the concrete blocks. The heliport is constructed so that the upper part sticks out above the sea water face, and it is fixed to the sea bottom by respectively a plurality of rotary penetrating steel pipe piles 5 or the like so that a specified clearance 4 can be formed between the under face and the sea bottom face. In this way, the sea water circulates and natural light reaches the sea bottom. A joint plate 7 is laid between the blocks 2 in the runway 20, and covered with topsoil 6, and the like hardly breeds and proliferates and further, which can easily catches agricultural chemicals and herbicides and decomposes them and increases the working efficiency and decreasing the construction cost by using a composite culvert material keeping the water permeability for a long time regardless of weather and climate.