Disclosed is a switching power-supply device that induces a pulse voltage in a secondary winding of the transformer, and outputs, to a load, an output voltage rectified and smoothened by a secondary-side rectification smoothing circuit including a rectifier diode and a smoothing capacitor. An input-voltage detection circuit detects the input voltage of the alternating-current power supply. An error amplifier compares the output voltage with a reference voltage, and transmits a resultant error voltage to a primary side as a feedback signal. An internal oscillation circuit selects any of frequency decrease settings according to the input voltage detected by the input-voltage detection circuit, and performs, by using the selected frequency decrease setting, a frequency decreasing function of decreasing a switching frequency of the switching element at a light load, in response to the feedback signal transmitted from a secondary side to inform a state of the load.