A power supply system comprising: a switching power supply configured to rectify and smooth an alternating current voltage of an alternating current power supply and generate a predetermined direct current voltage; a first capacitor including a first electrode (C4p1) and a second electrode (C4p2), the first electrode (C4p1) being connected to one end of the alternating current power supply; a second capacitor including a third electrode (C5p1) and a fourth electrode (C5p2), the third electrode (C5p1) being connected to the other end of the alternating current power supply; a rectification circuit configured to rectify an alternating current voltage applied to the first and second capacitors (C4, C5); the second and fourth electrodes (C4p2, C5p2) being connected to the rectification circuit, the rectification circuit including an output line connected with an output line of the switching power supply through a backflow regulation element provided on the output line of the rectification circuit, a power supply line positioned at a more downstream side than the backflow regulation element and connected to the output line of the switching power supply and the output line of the rectification circuit; an electricity storage unit connected to the power supply line; a control unit that is positioned at a more downstream side than the electricity storage unit, power being fed to the control unit through the power supply line, and characterized by the backflow regulation element being configured to regulate backflow of current from the switching power supply towards the rectification circuit; a signal generation circuit connected to the output line of the rectification circuit, the signal generation circuit being configured to generate a detection signal corresponding to a zero cross point of the alternating current power supply based on an output of the rectification circuit the signal generation circuit including a voltage limit element configured to limit a voltage of the output line of the rectification circuit to be lower than a voltage of the power supply line, and in that the control unit is configured to detect a zero cross point based on the detection signal (Sp). a switching power supply configured to rectify and smooth an alternating current voltage of an alternating current power supply and generate a predetermined direct current voltage; a first capacitor including a first electrode (C4p1) and a second electrode (C4p2), the first electrode (C4p1) being connected to one end of the alternating current power supply; a second capacitor including a third electrode (C5p1) and a fourth electrode (C5p2), the third electrode (C5p1) being connected to the other end of the alternating current power supply; a rectification circuit configured to rectify an alternating current voltage applied to the first and second capacitors (C4, C5); the second and fourth electrodes (C4p2, C5p2) being connected to the rectification circuit, the rectification circuit including an output line connected with an output line of the switching power supply through a backflow regulation element provided on the output line of the rectification circuit, a power supply line positioned at a more downstream side than the backflow regulation element and connected to the output line of the switching power supply and the output line of the rectification circuit; an electricity storage unit connected to the power supply line; a control unit that is positioned at a more downstream side than the electricity storage unit, power being fed to the control unit through the power supply line, and characterized by the backflow regulation element being configured to regulate backflow of current from the switching power supply towards the rectification circuit; a signal generation circuit connected to the output line of the rectification circuit, the signal generation circuit being configured to generate a detection signal corresponding to a zero cross point of the alternating current power supply based on an output of the rectification circuit the signal generation circuit including a voltage limit element configured to limit a voltage of the output line of the rectification circuit to be lower than a voltage of the power supply line, and in that the control unit is configured to detect a zero cross point based on the detection signal (Sp).