PURPOSE: To accurately detect out-of-synchronism over a wide frequency range by providing a reference voltage converting means which changes a reference voltage when output frequency information is switched in an interlocking way and window comparator means which sends an out-of-synchronism warning when a control voltage becomes to have a window width other than a prescribed one.

CONSTITUTION: The reference voltage converting means 2 converts output frequency information, namely, information regarding the output frequency of a voltage-controlled oscillator VCO into a reference voltage having a prescribed window width and supplies the reference voltage to the window comparator means 3. Since the control voltage of the voltage-controlled oscillator VCO is also applied across the means 3, a warning is sent when the control voltage gets out of the window width. Moreover, when the output frequency information is switched, the reference voltage is also switched correspondingly, but the window width is kept as it is. Therefore, out-of-synchronism can be detected accurately even when the output frequency of the voltagecontrolled oscillator VCO changes in a wide range.