A common mode choke coil includes a core, and first and second coils opposed to each other and wound on the core. The core can have a square shape, or an elongated shape having a long axis and a short axis when viewed in a direction along a central axis of the core. Each of the first and second coils is a single-layer coil. An area of a cross-section of the core taken perpendicular to a circumferential direction of the core is constant in the circumferential direction of the core. The cross-section of the core has a quadrilateral shape.