A stable power supply apparatus in accordance with the present invention comprises a secondary battery, a bidirectional chopper circuit and a bidirectional converter, wherein the secondary battery, the chopper circuit and the converter are connected in this order in the direction from the secondary battery side to a system bus line side. The converter is formed of a wide-gap semiconductor device, more particularly, a wide-gap bipolar semiconductor device, and the instantaneous large-power operation capability of the wide-gap bipolar semiconductor device and the instantaneous large-power supplying capability of the secondary battery are utilized. For a short time during which the influence of an instantaneous drop is prevented, the converter is operated as a converter having capability exceeding the instantaneous large-power supplying capability of the secondary battery and having power capacity several times or more the rating of the converter.