A power supply system (1, 1#) having a plurality of power storage units (BAT1, BAT2 to BATN) each configured to be capable of charge and discharge, comprising: a power line (MPL, MNL) configured to be capable of supplying and receiving electric power between a load device and said power supply system (1, 1#); and a plurality of voltage conversion units (CONV1, CONV2 to CONVN) provided between respective ones of said plurality of power storage units (BAT1, BAT2 to BATN) and said power line (MPL, MNL), each performing a voltage conversion operation between corresponding said power storage unit and said power line (MPL, MNL); characterized by an operation mode selection unit selecting an operation mode in which a voltage conversion operation of one voltage conversion unit out of first and second voltage conversion units (CONV1, CONV2) included in said plurality of voltage conversion units (CONV1, CONV2 to CONVN) is allowed and a voltage conversion operation of another voltage conversion unit is stopped, in accordance with an electric power request from said load device; and a voltage conversion unit selection unit selecting said voltage conversion unit to be allowed to perform the voltage conversion operation based on output voltages of respective corresponding said power storage units (BAT1, BAT2), when said operation mode is selected, and said voltage conversion unit selection unit switching between said voltage conversion units (CONV1, CONV2) to be allowed to perform the voltage conversion operation, when the output voltage of said power storage unit (BAT1, BAT2) corresponding to said voltage conversion unit (CONV1, CONV2) being performing the voltage conversion operation is lower than the output voltage of said power storage unit (BAT1, BAT2) corresponding to said voltage conversion unit of which voltage conversion operation has been stopped by an amount exceeding a prescribed threshold voltage. a power line (MPL, MNL) configured to be capable of supplying and receiving electric power between a load device and said power supply system (1, 1#); and a plurality of voltage conversion units (CONV1, CONV2 to CONVN) provided between respective ones of said plurality of power storage units (BAT1, BAT2 to BATN) and said power line (MPL, MNL), each performing a voltage conversion operation between corresponding said power storage unit and said power line (MPL, MNL); characterized by an operation mode selection unit selecting an operation mode in which a voltage conversion operation of one voltage conversion unit out of first and second voltage conversion units (CONV1, CONV2) included in said plurality of voltage conversion units (CONV1, CONV2 to CONVN) is allowed and a voltage conversion operation of another voltage conversion unit is stopped, in accordance with an electric power request from said load device; and a voltage conversion unit selection unit selecting said voltage conversion unit to be allowed to perform the voltage conversion operation based on output voltages of respective corresponding said power storage units (BAT1, BAT2), when said operation mode is selected, and said voltage conversion unit selection unit switching between said voltage conversion units (CONV1, CONV2) to be allowed to perform the voltage conversion operation, when the output voltage of said power storage unit (BAT1, BAT2) corresponding to said voltage conversion unit (CONV1, CONV2) being performing the voltage conversion operation is lower than the output voltage of said power storage unit (BAT1, BAT2) corresponding to said voltage conversion unit of which voltage conversion operation has been stopped by an amount exceeding a prescribed threshold voltage.