PURPOSE: To provide a three-phase transformer with which the mutual interference between a three-phase load system and a single-phase load system can be reduced when single-phase power is fed together with three-phase power.

CONSTITUTION: The prescribed end parts of three-phase secondary windings C2u to C2w are phase-connected, three-phase secondary windings are delta- connected, and three-phase secondary side terminals V2, V2 and W2 are led out from the three-phase connected parts of the three-phase secondary windings. A tertiary winding C3u for single phase load driving is provided on a phase. A mid-point tap O is led out from the mid-point of the tertiary winding C3u, and the mid-point tap O of the tertiary winding C3u is connected to the phase connection part of the secondary windings C2v and C2w of the other two phases.