To reduce the swinging of a moored float caused by waves by providing a compensator composed of a cylinder or the like for adjusting the length of a sway reducing cable to be variable and controlling the sway reducing cable based on the redetected values of a cylinder air pressure, a sway reducing cable tension and the pulling out amount of the sway reducing cable.

A compensator is composed of a cylinder 19 and a sieve 22 for adjusting the tension of the tip of a rod 21. A sway reducing cable 15 is hung on tension adjusting, tension receiving and suspending sieves 22, 23 and 17 and connected to the heavy bob 18 of a water bottom 13. A tension sensor 24 for detecting the tension of the sway reducing cable 15 is provided in the tension receiving sieve 23 and a pulled out length sensor 25 for detecting the pulling out amount of the sway reducing cable 15 is provided between the tension receiving sieve 23 and the suspending sieve 17. An air pressure sensor 39 for detecting a supply pressure is connected to an air supply line 37. A control means 40 controls a pulling out amount so as to set the tension of the sway reducing cable 15 within a specified range based on the detection values of the sensors 24, 25 and 39 and thus the swinging of a float 10 is reduced.