A magnetic circuit for a planar diaphragm type loudspeaker having a square planar diaphragm and square voice coil in which four magnetic circuits are arranged in a square corresponding to the edges of the planar diaphragm. Pairs of plates are disposed opposite to one another upon which are attached a number of magnets with the opposed poles opposite one another along the inner walls of the oppositely disposed plates. Each of the magnets is rectangular in shape. A yoke connects the pairs of plates. Holes for permitting air flow are formed either in the yoke or in the pairs of oppositely disposed plates in patterns such as to not disturb the magnetic field. The magnetic circuit units made up by the magnets, pairs of plates, and yokes are joined at their ends by magnetic couplings. An electrical conductive member may be disposed over portions of the magnets to prevent the occurrence of third harmonic distortion due to current distribution.