PURPOSE: To eliminate disadvantages caused by obstacles of unnecessarily raised part by positioning the front end of a columnar body erected from the water bottom below the water level and connecting a cylindrical body which is lifted and lowered, while guiding the columnar body to a horizontally floating pontoon.

CONSTITUTION: The front end of a columnar body 1 erected from the water bottom is positioned below the water level, and a cylindrical body 2 opened at the lower end to guide the outer peripheral face of the columnar body 1 is inserted in the columnar body downward from above to connect the main body 3 of the pontoon to the upper end of the cylindrical body 2. The cylindrical body 2, fitted to the columnar body 1 moves vertically according to the rising and falling movement of the water level to release a user from such bothersome works as in conventional cases, in which a pontoon is moored to a vertical bar projected high above the water level by a string or the like and anchors are dropped crosswise.