To easily manufacture a porous high-voltage electret.

A die plate 200 is a ceramic plate where a plurality of holes 210 are regularly located, and a punch plate 100 is also a ceramic plate having a plurality of punches 110 to be fit with the holes of the die plate 200. To open holes in an electrode 20 and an electret 40 by means of the punch plate 100 and the die plate 200, the electrode 20 and the electret 40 which are integrally formed are placed on the die plate 200. Then, when the punch plate 100 is pressed against the electret 40, portions through which the punches 110 have passed in the electrode 20 and the electrets 40 are punched out and the punches 110 penetrating through the electret 40 and the electrode 20 are fit with the holes 210 of the die plate 200 so that holes 2 are opened in the electrode 20 and the electret 40.