Current sensor

A current sensor comprising:

• a magnetic core having a groove portion where a flat plate-like conductor through which a current to be measured flows is inserted, with a direction of plate width aligned with a direction of depth of the groove portion;
• and
• a detector element that is arranged more closely to an opening of the groove portion than the conductor in the groove portion, with a direction of detection laid along a direction of a space of the groove portion, and that detects strength of a magnetic field;
• wherein the core has a U-shaped portion in which a surface facing the conductor is formed parallel to a surface defining the plate width of the conductor, and narrow-width portions in which a groove width along the direction of the space of the groove portion gradually expands and a core width along the direction of the space of the groove portion gradually narrows, from both ends on the side of the opening of the U-shaped portion toward the side of the opening, and
• boundary portions between the U-shaped portion and the narrow-width portions are provided at a position facing the conductor.