The present invention provides a planar acoustic transducer including a vibrating diaphragm including a spiral voice coil provided on both surfaces or one surface of an insulating base film; and a permanent magnet corresponding to the voice coil, wherein the vibrating diaphragm includes at least a layer formed of a resin foam. The resin foam may be a polyethylene naphthalate (PEN) resin foam sheet which exhibits good vibration transmission whilst producing high sound pressure and rigidity without any increase in weight.