A modulation-signal generating circuit includes a temperature monitoring unit that detects a casing temperature of the circuit, a voltage control oscillator including two variable impedance circuits that independently control oscillation frequency based on an input control voltage, a frequency-correction-voltage generating unit that outputs a voltage for compensating for a temperature drift of an oscillation frequency according to the casing temperature detected by the temperature monitoring unit, to one of the variable impedance circuits, and an FM-modulation-voltage generating unit that outputs a modulation voltage containing a constant DC component not depending on temperature and a predetermined AC component, to the other variable impedance circuit, under a temperature drift compensation condition of the frequency-correction-voltage generating unit.