PURPOSE: To attain high speed and stable circuit operation by providing a temperature sensor outputting a voltage depending definitely on temperature and odd number stage gates receiving an output of a latch circuit and forming the circuit so that the output of the odd number stage gates is received by a clock selection circuit as its data input.

CONSTITUTION: In the case of selecting three kinds of clock frequencies in response to three temperature ranges, an odd number stage gate circuit 24 receives a latch output signal fLS70 and outputs output signals fS172, fS274, fS376 being signals specifying the 3 kinds of frequencies. In such a case, the frequency (f) of a clock signal fS78 is respectively different but generates proper clock frequency from three low, medium and high temperature ranges depending on a temperature T of a temperature sensor 10. An operationable fastest clock frequency is supplied to an integrated circuit chip to operate the said chip at a faster speed. Moreover, the clock frequency is fixed for a prescribed time to make the circuit operation stable at the frequency changeover.