A system for protecting against overheating of a CPU whose operation frequency is changed due to voltage variation of a driving power, includes a CPU temperature sensor to detect the temperature of the CPU, a reference temperature sensor to detect the temperature of a circuit block or multiple circuits heated due to the operation frequency of the CPU, a comparing part to output a signal indicating a higher temperature of the temperature of the CPU and the temperature of the circuit block or multiple circuits by comparing temperature values detected by the respective CPU and reference temperature sensors, and a CPU power supply to supply a stepwise decreasing driving power to decrease the operation frequency of the CPU in a stepwise fashion when the temperature value increases based on the signal indicating the higher temperature indicated by the comparing part.