A specific region of a polylactic acid sheet is heated by a microwave. To allow the polylactic acid sheet to exhibit piezoelectricity in the thickness direction of the polylactic acid sheet, a high voltage is applied to the heated polylactic acid sheet in the thickness direction of the polylactic acid sheet, and thereby the screw axes of at least a part of the polylactic acid molecules are relatively aligned with the thickness direction. Then the polylactic acid sheet is rapidly cooled, and thereby the polylactic acid molecules are immobilized. The same step is executed for other regions of the polylactic acid sheet, and thereby piezoelectricity is imparted to a wide area of the polylactic acid sheet in the thickness direction. The resultant piezoelectric sheet is capable of exhibiting a high piezoelectricity in the thickness direction.