A cooling mechanism for a circuit board includes a board having electronic components disposed thereon, one or more heat radiation plates disposed on the board, a cooling fan for cooling the board and the electronic components disposed on the board by blowing air to at least one of the heat radiation plates, and a metal plate attached to one end of the board to ground the board, thereby preventing the leakage of electromagnetic radiation noises from the electronic components. Further, the metal plate includes a bent portion bent in L-shape along a lateral side of the board, and the cooling fan is so disposed at the inner side of the bent portion as to be perpendicular to the board. Furthermore, an air receiving surface of the heat radiation plate is so arranged as to be oblique to an axial blowing direction of the cooling fan.