PURPOSE: To prevent sinking of a pontoon and to improve corrosion resistance by a method wherein a sinker or an anchor is formed with a fish-gathering block group made of weight concrete having a specified specific gravity in saturated surface-dry condition, and is single-point-moored to a pontoon made of light aerated concrete of a specified specific gravity in saturated surface-dry condition.

CONSTITUTION: A pontoon 1 is manufactured by using light aerated concrete having a specific gravity in saturated surface-dry condition of 1.0 or less. A number of fish-gathering place blocks 4 made of weight concrete having a specific gravity in saturated surface-dry condition of 2.0 or more are arranged to form a sinker or an anchor. The pontoon 1 made of light aerated concrete is single-point-moored to the sinker or the anchor through a mooring line 6. This constitution enables prevention of sinking of the pontoon since the pontoon 1 has no structural empty chamber. Further, the anchor has a function as a fish-gathering place, and enables protection and breeding of fishes. Single-point mooring enables prevention of entanglement of the mooring line and facilitates maintenance and management.