A power supply system comprising: a plurality of batteries; a plurality of input/output adjustment apparatuses that are electrically connected between the plurality of batteries and a load; and a control apparatus that controls input/output of power to/from the plurality of batteries by controlling the plurality of input/output adjustment apparatuses, wherein: during motoring in which power is output from the plurality of batteries to the load via the plurality of input/output adjustment apparatuses, the control apparatus controls the plurality of input/output adjustment apparatuses to output the power with priority from a battery whose SOC exceeds an SOC average value (SOCave) of the plurality of batteries; and during the motoring, if a power amount capable of being output from the battery whose SOC exceeds the SOC average value is greater than a requested output (Poreq) of the load, the control apparatus controls the plurality of input/output adjustment apparatuses to output the power from this battery to the load via the input/output adjustment apparatus, or during the motoring, if a requested output of the load is greater than or equal to a power amount capable of being output from the battery whose SOC exceeds the SOC average value, the control apparatus controls the plurality of input/output adjustment apparatuses to output just the power amount from this battery to the load via the input/output adjustment apparatus, and to output an insufficient portion of the power amount relative to the requested output from a battery whose SOC is less than or equal to the SOC average value to the load via the input/output adjustment apparatus connected to the battery whose SOC is less than or equal to the SOC average value; and during regeneration in which power is input from the load to the plurality of batteries via the plurality of input/output adjustment apparatuses, the control apparatus controls the plurality of input/output adjustment apparatuses to input the power with priority to a battery whose SOC is less than or equal to the SOC average value. during motoring in which power is output from the plurality of batteries to the load via the plurality of input/output adjustment apparatuses, the control apparatus controls the plurality of input/output adjustment apparatuses to output the power with priority from a battery whose SOC exceeds an SOC average value (SOCave) of the plurality of batteries; and during the motoring, if a power amount capable of being output from the battery whose SOC exceeds the SOC average value is greater than a requested output (Poreq) of the load, the control apparatus controls the plurality of input/output adjustment apparatuses to output the power from this battery to the load via the input/output adjustment apparatus, or during the motoring, if a requested output of the load is greater than or equal to a power amount capable of being output from the battery whose SOC exceeds the SOC average value, the control apparatus controls the plurality of input/output adjustment apparatuses to output just the power amount from this battery to the load via the input/output adjustment apparatus, and to output an insufficient portion of the power amount relative to the requested output from a battery whose SOC is less than or equal to the SOC average value to the load via the input/output adjustment apparatus connected to the battery whose SOC is less than or equal to the SOC average value; and during regeneration in which power is input from the load to the plurality of batteries via the plurality of input/output adjustment apparatuses, the control apparatus controls the plurality of input/output adjustment apparatuses to input the power with priority to a battery whose SOC is less than or equal to the SOC average value.