PURPOSE: To reduce installation cost and to simply carry out maintenance- control by utilizing water head difference between overflow water temporarily staying in a staying section and the surface of the sea for discharging the overflow water as a jetting flow from the lower end opening part of a cylindrical guide member toward the bottom of the sea.

CONSTITUTION: A wave having arrived at the device body 12 of a floating system sea water alternating device 10 to float on the surface of the sea gets over the bent-swelled upper face of the device body 12 to turn into wave overtopping, and an overflow water caused by the wave overtopping flows int a staying section 14 as it is. When a height difference between the level of the overflow water having flowed into the staying section 14 and the surface of the sea, namely a waterhead difference H is caused, the overflow water stayed temporarily is discharged as a jetting flow from the lower end opening part 22 of a cylindrical guide member 18 toward the bottom of the sea by pumping action, and replaced with the sea water near the bottom of the sea, and the alternation of the sea water in the direction of depth is therefore carried out. Thus installation cost can be reduced, and maintenance control can be simply carried out.