PURPOSE: To accurately measure current by providing a core with a means that regulates secondary coil position, for even winding of the secondary coil, and preventing displacement of the secondary coil.

CONSTITUTION: A recessed part 7 is formed at the corner on the inside periphery side of a core 6, and a recessed part 8 is formed at the corner on the outside periphery side. A coil 4 is wound, while engaged with the recessed part 7 at the inside periphery part and with the recessed part 8 at the outside periphery part, so that a secondly coil 5 is formed. Thus, at the corners where the secondary coil 5 of inside periphery side and outside periphery side of the core 6 contacts, the recessed parts 7 and 8 are formed, and the secondary coil 5 is engaged inside, so that the secondary coil 5 is evenly wound, and the displacement after winding is prevented.