A communication transformer for power line communication includes a magnetic core, a plurality of transfer-purpose windings wound on the magnetic core, and an additional winding. The additional winding is wound on the magnetic core in such a manner that the additional winding is positioned between the plurality of transfer-purpose windings, and does not contribute in signal transfer operations. The communication transformer is arranged such that a half way portion of the additional winding along a length direction thereof is electrically connected to a constant potential.