PURPOSE: To improve the performance in frequency change against temperature change by inserting a heat-sensitive element between the connections of an output of an amplifier section of a piezoelectric oscillator and a feedback section.

CONSTITUTION: A piezoelectric oscillator 2 is accommodated in the inside of a temperature insulating section 1 of a thermostatic oven to decrease the temperature change as less as possible. The heat-sensitive element, e.g., a parallel circuit comprising a thermister 17 and a resistor 18 is inserted between the connections of an output of the amplifier section and a feedback section. When a forward voltage drop of diodes 15, 16 is changed by +ΔVF because of temperature change, the resistance value of the thermister 17 is changed by +ΔRTH and the combined resistance value of the thermister 17 and the resistor 18 is changed by +ΔR. In setting the amount of change +ΔR to a value to cancel the +ΔVF, the current value fed back to a piezoelectric vibrator 2 is not changed against temperature change. As a result, the exciting level of the piezoelectric vibrator 2 is kept constant.