PURPOSE: To drive a semiconductor device at a maximum frequency band for an ambient temperature by changing the oscillation frequency of an oscillator by a signal outputted from an ambient temperature measuring instrument.

CONSTITUTION: A temperature sensor 1 measures the ambient temperature of the semiconductor device 1 and a change in the ambient temperature of the device 1 is transmitted to the oscillator 3 through a signal line 5. The oscillator 3 changes the oscillation frequency in accordance with temperature information received from the sensor 4 so as to increase the frequency when the temperature is dropped or decrease it at the time of increasing the temperature, so that a clock with the maximum frequency for normally driving the device 1 is always supplied to the device 1 through a clock feeder 2.