PURPOSE: To make it unnecessary to work in advance a surrounding wall of a water tank to an irregular shape, and also to change easily a shape and a size in accordance with an acoustic characteristic of a purpose by forming the surrounding wall of the water tank body to a shape of a rectangular parallelepiped, and forming an irregular surface by a spray foam resin layer on its inside surface.

CONSTITUTION: Foam resin layers 6W10 consisting of independent foams sprayed to each inside surface of surrounding walls 1W5 of a water tank body constitute an air layer of the independent foams on a boundary surface of the inside surface of the water tank and water of the water tank, and become substance layers whose acoustic impedances are different remarkably, by which the reflection becomes large. Also, confronted surfaces of each resin layer 6W10 are not parallel surfaces but irregular, therefore, a diffusion sound field can be obtained, and its surface is waterproof completely since a waterproof paint 12 is applied on it. Also, each resin layer is sprayed at a job site, therefore, it is worked to a seamless and unified film, and also a shape and a size of the inside surface can be changed by changing its thickness, and an acoustic characteristic of a purpose can be obtained easily.