An uninterruptible power supply apparatus (1) drives a converter (6) at a relatively high frequency (fH) and supplies an output voltage (VDC) of the converter (6) directly to a load (23) when a load current (IL) is greater than a predetermined value (Ic), and drives the converter (6) at a relatively low frequency (fL), steps up the output voltage (VDC) of the converter (6) by a bidirectional chopper (11), and supplies the output voltage (VDC) of the converter (6) to the load (23) when the load current (IL) is smaller than the predetermined value (Ic).