To create a seaweed bed capable of protecting germs of seaweeds from a natural enemy, improving absorption of nutrient salt by generating a flow at a flow speed suitable for a place with the seaweeds and preventing the seaweeds from being separated to flow out by ocean waves.

This block is shaped like a plate or a semi-cylinder and provided with a number of a projecting and recessed part 2 having a polygonal or trapezoidal section on the top face, the upper width of the projecting part being 200 mm or less, the bottom width of a recessed part being 100 mm or less, the depth being 30 mm or more. The lower surface thereof is provided with plural legs 7 for forming an inhabitable space 8 for small animals, and a penetrating opening 6 is provided between the upper and lower surfaces.