To provide a concrete block for making a seaweed bed, capable of cultivating marine algae even in a withered seashore area or a sandy ground sea area, decreasing the eating pressure of a sea urchin and artificially cultivat ing and settling marine algae, considering a seaweed bed and fishes and being installed as a coated block for consolidation of foundation for a sand deposit and to provide a method for using the concrete block.

This concrete block for making a seaweed bed is obtained by providing a bottom plate 1 of an approximately quadrilateral reinforced concrete having a proper thickness with a reinforced concrete T-shaped body 7 having a proper thickness, width, height and approximately the same length as that of the bottom plate 1 and forming seaweed bed plates 3 on upper plates 2 of a structure of a row or of plural integrated rows and is set on the bottom of the sea in such a way that the length direction of the T-shaped body 7 is adjusted to the advance direction of waves.