PURPOSE: To synchronize the phase of an oscillation output signal with an external signal by providing a means applying a prescribed external signal to an input terminal of a crystal oscillator having a feedback circuit.

CONSTITUTION: In receiving a high level control signal from a terminal 51 for control signal input, a switching FET 5 is conductive and an external signal from a terminal 52 is fed to an input terminal of an inverter 3 via an inverter 4. Thus, the crystal oscillator 1 is excited by the external signal and an output signal of the inverter 3. In receiving a low level control signal from the terminal 51, the switching FET 5 is nonconductive and the crystal oscillation circuit comprising the crystal resonator 1 and the inverter 3 is formed and the oscilla tion is started. Since the oscillation excited through the external signal inputted from the terminal 52 is maintained in the crystal resonator 1 itself in this case, the output phase of the crystal oscillation circuit is made coincident with the phase of the external output signal.