To reduce tossing and pitching without changing a plane shape of a medium-small scale floating body structure like hindering ship operation for leaving from and arriving at a quay, to simplify a mooring device, to reduce weight, to reduce a mooring cost, to improve an operation rate, and to secure safety of equipment and human life.

The plane shape of this floating body structure (hereinafter referred to as a floating body) 1 is a rectangular shape having the length L in the incident direction of an incident wave 2 and a width W in the direction orthogonal to the incident direction of the incident wave. In a side surface shape in the lengthwise direction, draft d1 of a range of L1 in the lengthwise direction from the incident side edge 1a of the incident wave is set deeper than draft d2 of the other range L2 (L-L1), and ballast 4 is loaded in a part of the draft of d1 so that an upper surface 1b of the floating body 1 becomes parallel to the draft line 3.