A coaxial type planar diaphragm loudspeaker system in which a plurality of coaxially arranged polygonal diaphragms are provided with separate magnetic circuits. Each magnetic circuit includes two parallel plate along each side of the diaphragm with a voice coil bobbin having the same shape as the corresponding polygonal diaphragm coupled thereto with a voice coil attached to the opposite end and disposed in the gap formed in the magnetic circuit. The area of the diaphragm falling within the juncture line between the voice coil bobbin and diaphragm is made equal to the area of the diaphragm outside the juncture line so as to eliminate split vibration.