A temperature detection circuit (18) detects temperature around an oscillation circuit (20) equipped with voltage variable capacitors (23, 24). According to its temperature detection signal, a temperature compensation circuit (30) produces a voltage signal as a temperature compensation signal. That voltage signal is supplied to the voltage variable capacitors (23, 24) in order to sustain oscillation frequency of the oscillation circuit (20) at a substantially constant level. When a switch element (1) is, turned on by a non-TCXO signal, both terminals of each voltage variable capacitor (23, 24) are brought to the same potential (ground potential). Consequently, each voltage variable capacitor has a predetermined capacitance and temperature compensation function is made ineffective. In that state, initial frequency regulation is performed under normal temperature.