A switching power source comprising: a transformer; switching means configured to switch a voltage supplied to a primary side of the transformer according to a plurality of pulses for turning on the switching means; output means configured to output a voltage generated on a secondary side of the transformer; control means configured to control an operation of the switching means based on the voltage generated on the secondary side of the transformer; and OFF-period setting means (130, 131) configured to set an OFF-period of the switching means so that during light load running, where the voltage generated on the secondary side of the transformer is lower than the voltage generated in normal load running, a harmonic of a sound wave generated from the transformer is attenuated, wherein during light load running the switching means is switched to a burst mode operation to lower the voltage generated on the secondary side of the transformer, and the OFF-period setting means (130, 131) sets the OFF-period of the switching means in the burst mode operation, and the burst mode operation comprises groups of pulses having a short interval between rising edges of neighboring pulses in each group and a relatively longer interval between each group of pulses, characterized in that during the burst mode operation, an operation to turn on the switching means twice in a predetermined cycle is performed, and the OFF-period setting means sets the OFF-period of the switching means to set an interval of turning on the switching means twice, the OFF-period setting means (130, 131) includes a time constant circuit (906, 905) having a resistor and a capacitor which are configured to set the short interval to half of a resonance cycle of the transformer and wherein the OFF-period setting means (130, 131) is arranged to output a signal according to the time constant circuit (906, 905) and, during light load running the control means is arranged to turn off the switching means according to the signal. a transformer; switching means configured to switch a voltage supplied to a primary side of the transformer according to a plurality of pulses for turning on the switching means; output means configured to output a voltage generated on a secondary side of the transformer; control means configured to control an operation of the switching means based on the voltage generated on the secondary side of the transformer; and OFF-period setting means (130, 131) configured to set an OFF-period of the switching means so that during light load running, where the voltage generated on the secondary side of the transformer is lower than the voltage generated in normal load running, a harmonic of a sound wave generated from the transformer is attenuated, wherein during light load running the switching means is switched to a burst mode operation to lower the voltage generated on the secondary side of the transformer, and the OFF-period setting means (130, 131) sets the OFF-period of the switching means in the burst mode operation, and the burst mode operation comprises groups of pulses having a short interval between rising edges of neighboring pulses in each group and a relatively longer interval between each group of pulses, characterized in that during the burst mode operation, an operation to turn on the switching means twice in a predetermined cycle is performed, and the OFF-period setting means sets the OFF-period of the switching means to set an interval of turning on the switching means twice, the OFF-period setting means (130, 131) includes a time constant circuit (906, 905) having a resistor and a capacitor which are configured to set the short interval to half of a resonance cycle of the transformer and wherein the OFF-period setting means (130, 131) is arranged to output a signal according to the time constant circuit (906, 905) and, during light load running the control means is arranged to turn off the switching means according to the signal.