An oscillating signal generating device includes: an oscillating circuit arranged to generate an oscillating signal according to a current controlled signal; and a control signal generating circuit coupled to the oscillating circuit, the control signal generating circuit for receiving a first reference voltage and a second reference voltage, the control signal generating circuit operated between the first reference voltage and the second reference voltage, and the control signal generating circuit arranged to generate the current controlled signal according to a voltage input signal; wherein the control signal generating circuit is capable of monotonically generating the current controlled signal according to the voltage input signal when a voltage level of the voltage input signal falls between the first reference voltage and the second reference voltage.