A cooling device for a portable electronic comprises a radiation plate and a fin for radiating the heat from the CPU. When the plug of an external power. supply is attached to the body of the portable electronic, the CPU may operate at a high clock frequency. The cooling device achieves a high cooling performance by operating a ventilation fan directed to the fin, for example. When the plug is removed from the body of the portable electronic, the CPU may operate at a low clock frequency. Power consumption can be reduced, so that the electric power stored in the built-in battery can be saved. The duration time of the operation can be extended. Calorific power generated at the CPU can also be reduced, so that the cooling device is adapted to suppress the rise in the temperature of the CPU without operation of the ventilation fan.