a power supply system equipped with a power generating unit including an alternating - current generator and a rectifier for rectifying the generated output of the alternating - current generator . in addition , a switching relay is provided between a low - voltage load and a high - voltage load for selectively supplying the generated output from the power generating unit to one of the low - voltage load and high - voltage load . a field current control unit controls a field current flowing into a field winding of an ac generator and a high - voltage load control unit controls a switching operation of the switching relay and further controls the field current control unit in accordance with a state of the high - voltage load . the high - voltage load control unit operates the power generating unit through the field current control unit in switching between the low - voltage load and the high - voltage load so that the generated output is applied to the switching relay , thus removing oxide films on a surface of the switching relay to ensure the conductivity in the switching relay .