A switching power supply unit is provided which provides improved response for frequency switching with a smooth rise in voltage. The switching power supply unit includes: a rectifier circuit for rectifying an alternating current from an AC power source into a direct current; a switching circuit for switching the current rectified by this rectifier circuit using a switching device; a pulse oscillator circuit for outputting a switching signal to the switching device; and a transformer circuit for stepping a voltage up or down depending on the current switched by this switching circuit. A frequency switching unit is also used to detect a pulse output from the switching circuit. Based on the state of this pulse output, the frequency switching unit changes a resistance using resistors, thereby switching the frequency of the switching signal in the pulse oscillator circuit.