A core is divided by alternately arranging plural magnetic material portions and plural non-magnetic material portions in a circumferential direction of the core through which a primary conductor penetrates. A conductor is wound around the core under conditions in which each core cross section of the core intersects the magnetic material portion and the non-magnetic material portion, each core cross section including a cut end surface of each conductor of a secondary winding wound around the core, and a ratio of a magnetic material portion cross-sectional area of the magnetic material portion to a non-magnetic material portion cross-sectional area of the non-magnetic material portion at the core cross section is kept constant at each core cross section.