PURPOSE: To obtain a zero phase transformer wherein the height and width are reduced as compared with the conventional one, without exerting adverse influence on balancing characteristics.

CONSTITUTION: A ring type core is formed by using magnetic material of high permeability, a coil is wound around the core, and the outer surface is covered with insulative material, thereby forming a zero phase current transformer. In this zero phase current transformer 1, the ring type outer shape of the core and the outer shape of an aperture are almost equilateral triangles, and cable insertion ports 9 whose shapes are almost equilateral triangles are formed in the aperture part of the core. Primary conductors 2a, 2b, 2c for three-phase system are inserted into the insertion ports.