According to the present invention, there is provided a semiconductor device including a power supply circuit which receives an external power supply voltage supplied, and outputs an internal power supply voltage not higher than the external power supply voltage; a system module which receives the internal power supply voltage, and performs a predetermined operation; and a performance monitor circuit which measures a processing speed of said system module when the internal power supply voltage is applied, and, on the basis of the processing speed, outputs a first control signal which requests to set the external power supply voltage at a first level, and a second control signal which requests said power supply circuit to set the internal power supply voltage at a second level. The power supply circuit outputs the internal power supply voltage having the second level on the basis of the second control signal applied thereto.