PURPOSE: To attain miniaturization and low power consumption by providing a DC conversion means converting temperature information into another temperature information having a polygonal line characteristic approximating a frequency temperature characteristic of a voltage controlled crystal oscillator, and sending the resulting information to a control voltage means.

CONSTITUTION: A DC conversion means 3 is provided between a temperature sensor 11 and a control voltage generating means 4, converts temperature information proportional to a temperature change outputted from the temperature sensor 11 into temperature information having a polygonal line characteristic approximated to a frequency temperature characteristic of a voltage controlled crystal oscillator 15 and the information is fed to the voltage controlled crystal oscillator 15 as a control voltage. Since a change in the temperature information is large when the frequency change with respect to a minute temperature range T is large and the change in the temperature information is small when the frequency change with respect to a minute temperature range T is small, only nearly required temperature information is fed to the control voltage generating means. Thus, miniaturization and low power consumption are attained.