Swiching power supply device with switchable output voltage setting and integrated circuit for swiching power supply device

[Problem] To provide a switching power supply device capable of switching to a predetermined output voltage and maintaining high efficiency. [Solution] A switching power supply device that switches the setting of an output voltage thereof on the basis of an external signal is characterized by being equipped with: a transformer T1 having a primary winding P1 and secondary windings S1, S2; multiple synchronous rectification components SR1, SR2 corresponding to the number of the secondary windings, which are disposed in a secondary-side rectification section for rectifying respective pulse voltages of the secondary windings; a switch component SW2 for switching between the secondary windings S1, S2; and a control circuit 20 for on-off control of the synchronous rectification components SR1, SR2 and for turning on or off the switch component SW2, whereby the switch component SW2 is turned on or off according to whether the level of a voltage value set for an output voltage Vo is high or low, thereby selecting either one of the synchronous rectification components so as to perform synchronous rectification of the pulse voltage of the corresponding secondary winding, and when the output is stopped with the output voltage Vo being set to the high setting value, synchronous rectification component SR2 of the synchronous rectification components, which is located on the side for outputting the output voltage Vo having the high setting value, is caused to perform a switching operation so as to remain on until the output voltage Vo drops to the low voltage setting value.