Abstract: In an apparatus for parallel operation of plural uninterruptable power source devices, each power source device includes at least one of an operation mode monitor for a logic operation on the output of the AC input monitoring circuit of each of the uninterruptable power source devices to feed the result thereof as a mode controlling signal to the mode controller, a parallel synchronizing signal generator for a logic operation on the output of an oscillating circuit which drives the invertor in each of the uninterruptable power source devices and, on detection of abnormality in the AC input, feeds the result thereof as a trigger pulse to the oscillating circuit, and a synchronization monitor for a logic operation on the output from the oscillating circuit in each of the uninterruptable power source devices and, when the differences between the phase of the output from the oscillating circuit of one of the triport uninterruptable power source devices and that from the oscillating circuits of remaining triport u