An object of the present invention is to provide a power supply voltage stabilizing method that can suppress the performance of switching power supply from being deteriorated even when a battery voltage varies and/or load conditions change. In a power supply voltage stabilizing method of a switching power supply including an output power MOS to which a battery voltage is supplied and a PWM feedback control unit that controls the output power MOS, the PWM feedback control unit includes a voltage feedback controller that controls on the basis of a power supply voltage output from the switching power supply and a current feedback controller that controls on the basis of a current output from the switching power supply. A variation in the battery voltage and/or a change in the load condition of the switching power supply are/is detected, and the bandwidth of the PWM feedback control unit is dynamically changed in accordance with the result of the detection.