According to one embodiment, a heat sink includes an extended end screwed to a printed circuit board. The extended end includes a screw hole and an engagement projection. A notch is formed by cutting the side of the printed circuit board to a position at which the notch does not interfere with signal lines. The notch is engaged with an engagement projection extending from the extended end, thereby preventing the heat sink from rotating when the heat sink is screwed to the printed circuit board.