A current detection busbar has a penetrating portion that penetrates a hole portion of a magnetic core and two flat plate-like terminal portions that are respectively continuous with opposite sides of the penetrating portion. The terminal portions have a larger width and a smaller thickness than the penetrating portion. An insulating casing has busbar holes that are penetrated by the respective terminal portions of the current detection busbar. An edge portion of each busbar hole is constituted by flat surfaces that face the terminal portion with a gap left between each flat surface and the terminal portion, a plurality of projecting portions that sandwich the terminal portion while coming into contact with the front and back surfaces of the terminal portion, and curved surfaces that face respective corner portions of the terminal portion with a gap left between each curved surface and the corresponding corner portion.