A power supply system includes: a switching power supply (10); and a control device, wherein the switching power supply (10) includes: a transformer (23); a semiconductor switching element (25); a switch control unit (50); and a rectifying/smoothing circuit (21), the switch control unit is supplied with power from the main power supply (15), starts to control the switching of the semiconductor switching element (25), and oscillates a primary side (N1) of the transformer (23) to induce a voltage on the secondary side (N2) of the transformer (23), the control device outputs a control pulse signal to the switch control unit (50) to stop the oscillation of the transformer (23), thereby changing the mode of the switching power supply to an output stop mode, in an output mode in which the switching power supply smoothes the voltage induced on the secondary side and outputs the smoothed voltage.