PURPOSE: To expand displacement without lowering flatness.

CONSTITUTION: A horn body 1 is composed of an input side horn 4 on the input surface 2 side and a radiation side horn 5 on the radiant surface 3 side. The input side horn 4 and the radiation side horn 5 are joined by screwing a male screw section 4a into a female screw section 5a, and the joining sections are brought to the state in which a sound is passed acoustically approximately completely. The specific gravity of the material of the input side horn 4 is made larger than that of the material of the radiation side horn 5, and the acoustic velocity of the material of the horn 4 is made faster than that of the material of the horn 5. Accordingly, even when the area of a radiant surface 3 is made the same as that of an input surface 2, the displacement of the radiant surface 3 to the input surface 2 is expanded.