To provide a highly stable oscillation frequency control circuit wherein the frequency thereof is corrected, an adequate range of the input levels of external reference signals is determined in accordance with temperature characteristics in detecting the external reference signal, and the control voltage to a VCO is controlled within and outside the adequate range. An oscillation frequency control circuit includes a selection switch that connects the phase comparator to the loop filter in an external reference synchronization mode and that connects the fixed voltage supplying circuit to the loop filter in a fixed voltage mode, and a CPU that switches the selection switch to the external reference synchronization mode or to the fixed voltage mode based on whether the detected voltage of an external reference signal level is within or outside of the adequate range.