PURPOSE: To suppress undesired frequency fluctuation during synchronization by inhibiting temperature compensation when a phase locked loop circuit network is in the synchronization state.

CONSTITUTION: When the phase locked loop circuit network is in the asynchronization state, a control information changeover circuit 4 selects control information from a digital temperature compensation circuit 2 nd gives, the selected information to a voltage controlled oscillator 1, in which the output frequency in the self-running state of the phase locked loop circuit network is made stable with temperature compensation. On the other hand, when the phase locked loop circuit network is in the synchronization state, the circuit 4 selects the synchronization information received from the phase locked loop circuit network depending on the information representing the synchronization state outputted from the circuit 3 and sends the information to the oscillator 1, which conducts frequency control following to only the synchronization information from the phase locked loop circuit network. In this case, the control information outputted from the circuit 2 is not delivered to the oscillator 1, then no temperature compensation is executed to suppress undesired frequency fluctuation during the synchronization.