To elongate an electric device in service life by a method wherein a silicon oxide electret layer is formed on the surface of an electrode formed of metal or high-concentration impurities, and a semiconductor layer which is lower in resistivity than a high-concentration impurity semiconductor is provided between the electrode and a silicon oxide electret layer.

When a semiconductor layer 12 is formed between an electrode 14 and a silicon oxide electret layer 10, an electric field generated between the electrode 14 and the silicon oxide electret layer can be relaxed, so that an electret device long in service life can be obtained. Furthermore, when a semiconductor layer 12 is formed between the electrode 14 and the silicon oxide electret layer 10, and an interface between the semiconductor layer 12 and the silicon oxide electret layer 10 is capable of trapping electric charge, so that charge trapped in the silicon oxide electret layer 10 and another charge trapped in the interface are made to act together, and it is considered that an electret device can be elongated in service life.