A power supply system comprising: a switching power supply to be connected to an AC power supply, the switching power supply being configured to rectify and smooth an AC voltage to generate predetermined DC voltage; a first capacitor including a first electrode (C1p1) connected to one end of the AC power supply, and a second electrode (C1p2) connected to a first contact node; a second capacitor including a first electrode (C2p1) connected to the other end of the AC power supply, and a second electrode (C2p2) connected to a second contact node; a rectifier circuit connected between the first contact node (Ndl) and the second contact node; and an output-signal generating circuit which is connected to the rectifier circuit, and which uses the rectified current (Irc) to generate a predetermined output signal, characterized by a surge-voltage suppressing circuit configured to suppress a surge voltage, wherein the surge-voltage suppressing circuit is connected between the first contact node (Ndl) and the second contact node, wherein the surge-voltage suppressing circuit comprises a first surge-voltage suppressing circuit and a second surge-voltage-suppressing circuit, wherein the first surge voltage-suppressing circuit is composed of a parallel circuit of a capacitor and a pair of bidirectional Zener diodes (ZD1, ZD3), the first surge-voltage suppressing circuit being connected between the first contact node and a reference potential, and wherein the second surge voltage-suppressing circuit is composed of a parallel circuit of a capacitor and a pair of bidirectional Zener diodes (ZD2, ZD4), the second surge-voltage suppressing circuit being connected between the second contact node and the reference potential. a switching power supply to be connected to an AC power supply, the switching power supply being configured to rectify and smooth an AC voltage to generate predetermined DC voltage; a first capacitor including a first electrode (C1p1) connected to one end of the AC power supply, and a second electrode (C1p2) connected to a first contact node; a second capacitor including a first electrode (C2p1) connected to the other end of the AC power supply, and a second electrode (C2p2) connected to a second contact node; a rectifier circuit connected between the first contact node (Ndl) and the second contact node; and an output-signal generating circuit which is connected to the rectifier circuit, and which uses the rectified current (Irc) to generate a predetermined output signal, characterized by a surge-voltage suppressing circuit configured to suppress a surge voltage, wherein the surge-voltage suppressing circuit is connected between the first contact node (Ndl) and the second contact node, wherein the surge-voltage suppressing circuit comprises a first surge-voltage suppressing circuit and a second surge-voltage-suppressing circuit, wherein the first surge voltage-suppressing circuit is composed of a parallel circuit of a capacitor and a pair of bidirectional Zener diodes (ZD1, ZD3), the first surge-voltage suppressing circuit being connected between the first contact node and a reference potential, and wherein the second surge voltage-suppressing circuit is composed of a parallel circuit of a capacitor and a pair of bidirectional Zener diodes (ZD2, ZD4), the second surge-voltage suppressing circuit being connected between the second contact node and the reference potential.