The present invention includes: a transformer; a primary source for supplying an alternating voltage of at least two frequencies to a primary winding of the transformer; and a filter circuit connected to a secondary winding of the transformer and having an attenuation that varies according to the frequency of the alternating voltage, where the frequency of the alternating voltage supplied to the primary winding of the transformer is controlled so that a desired alternating voltage is outputted from the filter circuit connected to the secondary winding of the transformer.