To provide a floating body type base structure having an excellent swing characteristic in waves.

This floating body type base structure 1 erectly supports a power generation system 2 on the ocean. The base structure 1 has a cylindrical main floating body 3 long in the axial direction wherein the central axis is vertically disposed to erectly support the power generation system 2, and a plurality of sub floating bodies 4 integrally provided with the main floating body 3 by a truss 5 such that the sub floating bodies 4 surround the main floating body 3. A lower part of the main floating body 3 in an erectly supporting state of the power generation system 2 is submerged in the sea, and an upper part of the sub floating body 4 is positioned on the sea. A flat plate 6 having a diameter more than a cross section of the main floating body 3 is provided in a draft part not more than the draft line of the main floating body 3 as horizontally as possible. Thereby, the base structure 1 does not affected by the water depth, has no problem of a ocean landform at all, can suppress swing of the whole device caused by influence of wave external force, wind external force or the like even offshore wherein ocean wind speed tends to increase as small as possible, and allows construction and carriage on the ocean with a compact structure. Fish reef effect can be added to an lower part.