To provide a method and a device capable of forming a sufficient electret in a short time.

In this electret making method, in the state of clamping a porous body to be processed with a pair of dielectrics whose total capacitance is 2×10-7 C/(m2.V) or more and total of thickness is 0.2 mm or more, a DC high voltage is applied between the dielectrics. The electret making device is provided with a means capable of clamping the porous body with the pair of the dielectrics whose total capacitance is 2×10-7 C/(m2.V) or more and total of thickness is 0.2 mm or more, and a means capable of applying the high DC voltage between the dielectrics.