An oscillating signal generating device includes an oscillating circuit and a control circuit. The oscillating circuit includes: a resonator having a first terminal and a second terminal for generating an oscillating signal, a resistive element having a first terminal coupled to the first terminal of the resonator, and a second terminal coupled to the second terminal of the resonator, and an oscillating start-up circuit having an input terminal coupled to the first terminal of the resonator and an output terminal coupled to the second terminal of the resonator. The control circuit generates a control signal to change the oscillating start-up circuit into a disable mode from an enable mode when the oscillating circuit generates an oscillating output signal under an operation mode, and outputs the oscillating signal generated by the resonator as the oscillating output signal of the oscillating circuit.