A structure and method for mounting a flat panel speaker are provided which are capable of reliably and accurately positioning a diaphragm in a cabinet and of improving an acoustic characteristic of the flat panel speaker. At least one first protruding portion formed in rectangular portions strikes a first side wall face of a corresponding corner portion of two side walls making up a front case and at least one second protruding portion formed in rectangular portions strikes a second side wall face of a corresponding corner portion of another side wall face of the front case, thus making it possible to properly position the diaphragm relative to the front case and, with the diaphragm being properly positioned, the diaphragm is attached to the front case with the gasket member bonded to the putting-face interposed between the diaphragm and the front case. The diaphragm comes into close contact with the front case in a manner to form a line-like boundary in a contacted portion.