PURPOSE: To ensure the low impedance to be shown to the oscillation loop by providing the active output means connected in series to the crystal resonator and then actuating the output means in response to the current flowing to the crystal resonator, thus producing the output signal having a high S/N.

CONSTITUTION: Crystal oscillator 14 is connected to the input terminal of inverse amplifier 16, and capacitor 12 is connected between the input terminal and reference line 24. In addition, capacitor 10 is connected between line 24 and feedback line 26 led from the output terminal of amplifier 16, and then line 24 is connected to the reference input terminal of amplifier 16. After this, the other terminal of resonator 14 is connected to one end of output capacitor 22 via base-earthed transistor (TR) 20. And the other end of capacitor 22 is connected to capacitor 10 and the base of TR20 each along with line 26. And capacitor 22 is set parallel substantially to resonator 14, and thus the current flowing to capacitor 22 is made identical to the current flowing the resonator 14. Thus the impedance is lowered to the oscillation loop, accordingly producing the signal having a high S/N.