An electric power saving transformer for three-phase current with an automatic voltage transferring device, in an autotransformer of a three-phase three-wire system or a three-phase four-wire system, has phase-winding of one set or plural sets of main coils which are connected to input terminals on a three-phase core-type core; connecting plural sets of exciting coils mutually connected in series to wind the three-phase core-type core between the ends of the main coils by means of plural combinations of connection; connecting plural electric switches between the ends of the main coils and the exciting coils which are switched to ON/OFF based on a voltage value detected at the input or output terminals; and connecting and disconnecting between the ends of the main coils and the exciting coils, by switching ON/OFF of the electric switches stepwise on the same value of voltages or currents between the phases in order to automatically change output voltage correspondingly to input voltage onto voltage transferring circuits in an uninterrupted manner.