The present invention relates to a frequency modulation device and a switching mode power supply using the frequency modulation device. To prevent electro-magnetic interference (EMI), it is required to slightly vary a switching operation frequency in an SMPS operation. In some embodiments, at least one first signal having a predetermined cycle is generated, a second signal corresponding to a level of a first signal is generated at a turn-off time of a switch, a first reference voltage having at least two different levels is generated according to the second signal, and an oscillator signal for increasing along a first slope during a first period and decreasing along a second slope during a second period between the first reference voltage and a second reference voltage having a level that is different from the first reference voltage is generated.