PURPOSE: To form easily a titled diaphragm, and also to increase its rigidity, by forming its sectional shape in the circumferential direction to a waveform, and also setting the number of waves of the outside circumferential part to two times or more of the center part.

CONSTITUTION: A section in the circumferential direction of a core material 4 is formed to a continuous waveform such as a trapezoidal wave, a triangular wave, etc. Also, a crest 4a of its waveform is made to branch to crests 4c, 4d at a point 4b on the way, and adjacent crests are connected by a plane 4g. As for the number of branches of the crest, two or more will do. By making the crest branch in this way, rigidity of the circumferential part can be raised, therefore, the pitch of the center part can be made larger than that of a fan- shaped core, and the forming is executed easily. Also, a skin material which is made to adhere to this core material 4 is divided into small areas in its circumference, therefore, a harmful resonance can be driven out of the band.