A power supply apparatus, which can detect an abnormal voltage drop in a power cell and, if recovery from this voltage drop is made, resume power generation, thereby realizing reasonable and stable power supply, is provided. The power supply apparatus comprises a combined power source composed of a plurality of power cells configured electrically independently, a switch for arbitrarily connecting each of the power cells by selectively connecting terminals of the power cells through switching elements, a voltage detector for respectively detecting differences in electrical potentials between the terminals of the power cells, and a controller for controlling the switch by a control signal to control ON/OFF states of the switching elements so as to isolate a power cell to halt a supply of electrical power therefrom when a voltage generated in the power cell has dropped to a set voltage or lower as found based on voltage signals which respectively indicate the differences in electric potentials detected by the voltage detector, and resume the supply of electrical power from the power cell whose power supply has been halted when a down time in the isolated power cell has reached a predetermined time or longer.