A square substrate is divided into four substrate areas with reference lines respectively passing through a central point of the substrate in parallel with one of diagonals of the substrate. A fin disposed between the substrate and an axial fan has plural louvers each facing one of the substrate areas and extending in a direction parallel to a corresponding reference line of the one of the substrate areas. Accordingly, when air is blown out from the axial fan toward the substrate, an air flow direction can be changed by the louvers so that the air radially flows out from the central point of the substrate. As a result, a heat spot is not produced on the substrate.