A frequency synthesizer comprising: a voltage controlled oscillating unit that outputs a frequency signal depending on a control voltage, a frequency divider that frequency-divides a frequency signal output from the voltage controlled oscillating unit, an analog/digital converting unit that digitizes a sinusoidal frequency-divided frequency signal output by the frequency divider to produce a digitized frequency signal, a vector extracting means that quadrature-detects the digitized frequency signal output by the analog/digital converting unit using a frequency signal for detection that is a digital signal and extracts a real part and an imaginary part of a complex expression of a vector rotating at a frequency corresponding to a frequency difference between the frequency signal for detection and the digitized frequency signal, a frequency difference extracting means that extracts a frequency difference between a frequency of the vector when a frequency of the frequency signal output by the voltage controlled oscillating unit becomes a set frequency and a frequency of the vector extracted in the vector extracting means, integration means (73, 8) that integrates the frequency difference to produce an integration value, a coupler that receives the integration value via a digital/analog converting unit and adds to it a frequency pull-in voltage to supply an addition result as the control voltage to the voltage controlled oscillating unit, and a frequency pull-in means for, when a Phase Locked Loop is locked and then the integration value deviates from a range set in advance, outputting the frequency pull-in voltage so that the Phase Locked Loop is not unlocked, characterized in that the frequency synthesizer further comprises: a monitoring means for monitoring either the control voltage to be input to the voltage controlled oscillating unit or a signal level of an input signal to the analog/digital converting unit; and a means for determining whether or not a level of the voltage monitored by said monitoring means deviates from a set range determined in advance and outputting an unlock detection signal when the level deviates from the set range. a voltage controlled oscillating unit that outputs a frequency signal depending on a control voltage, a frequency divider that frequency-divides a frequency signal output from the voltage controlled oscillating unit, an analog/digital converting unit that digitizes a sinusoidal frequency-divided frequency signal output by the frequency divider to produce a digitized frequency signal, a vector extracting means that quadrature-detects the digitized frequency signal output by the analog/digital converting unit using a frequency signal for detection that is a digital signal and extracts a real part and an imaginary part of a complex expression of a vector rotating at a frequency corresponding to a frequency difference between the frequency signal for detection and the digitized frequency signal, a frequency difference extracting means that extracts a frequency difference between a frequency of the vector when a frequency of the frequency signal output by the voltage controlled oscillating unit becomes a set frequency and a frequency of the vector extracted in the vector extracting means, integration means (73, 8) that integrates the frequency difference to produce an integration value, a coupler that receives the integration value via a digital/analog converting unit and adds to it a frequency pull-in voltage to supply an addition result as the control voltage to the voltage controlled oscillating unit, and a frequency pull-in means for, when a Phase Locked Loop is locked and then the integration value deviates from a range set in advance, outputting the frequency pull-in voltage so that the Phase Locked Loop is not unlocked, characterized in that the frequency synthesizer further comprises: a monitoring means for monitoring either the control voltage to be input to the voltage controlled oscillating unit or a signal level of an input signal to the analog/digital converting unit; and a means for determining whether or not a level of the voltage monitored by said monitoring means deviates from a set range determined in advance and outputting an unlock detection signal when the level deviates from the set range. a monitoring means for monitoring either the control voltage to be input to the voltage controlled oscillating unit or a signal level of an input signal to the analog/digital converting unit; and a means for determining whether or not a level of the voltage monitored by said monitoring means deviates from a set range determined in advance and outputting an unlock detection signal when the level deviates from the set range.