A power supply system for an electrically powered vehicle, the electrically powered vehicle incorporating a motor (MG1, MG2) for generating power to drive the vehicle and an inverter (14, 22) for driving said motor (MG1, MG2), the power supply system comprising: a main power storage device; an electric power feeding line for feeding electric power to said inverter (14, 22); a first voltage converter provided between said electric power feeding line and said main power storage device and configured to convert voltage therebetween bidirectionally; a plurality of sub power storage devices (BB1, BB2) provided in parallel to each other; a second voltage converter provided between said plurality of sub power storage devices (BB1, BB2) and said electric power feeding line, and configured to convert voltage between a sub power storage device selected from said plurality of sub power storage devices (BB1, BB2) and said electric power feeding line bidirectionally; a connection unit provided between said plurality of sub power storage devices (BB1, BB2) and said second voltage converter, and configured to connect and disconnect the selected sub power storage device to and from said second voltage converter; and a control device configured to control connection and disconnection of said selected sub power storage device to and from said second voltage converter, said control device including a disconnection determination unit configured to determine whether a disconnection request for disconnecting said selected sub power storage device from said second voltage converter should be generated based on a state of charge of said selected sub power storage device, when there is no new sub power storage device allowed to replace said selected sub power storage device, and the power supply system is characterized by the control device further including a disconnection prohibition unit configured to instruct said disconnection determination unit not to generate said disconnection request when a temperature condition regarding a temperature of at least one power storage device of said main power storage device and said plurality of sub power storage devices (BB1, BB2) is satisfied. a main power storage device; an electric power feeding line for feeding electric power to said inverter (14, 22); a first voltage converter provided between said electric power feeding line and said main power storage device and configured to convert voltage therebetween bidirectionally; a plurality of sub power storage devices (BB1, BB2) provided in parallel to each other; a second voltage converter provided between said plurality of sub power storage devices (BB1, BB2) and said electric power feeding line, and configured to convert voltage between a sub power storage device selected from said plurality of sub power storage devices (BB1, BB2) and said electric power feeding line bidirectionally; a connection unit provided between said plurality of sub power storage devices (BB1, BB2) and said second voltage converter, and configured to connect and disconnect the selected sub power storage device to and from said second voltage converter; and a control device configured to control connection and disconnection of said selected sub power storage device to and from said second voltage converter, said control device including a disconnection determination unit configured to determine whether a disconnection request for disconnecting said selected sub power storage device from said second voltage converter should be generated based on a state of charge of said selected sub power storage device, when there is no new sub power storage device allowed to replace said selected sub power storage device, and the power supply system is characterized by the control device further including a disconnection prohibition unit configured to instruct said disconnection determination unit not to generate said disconnection request when a temperature condition regarding a temperature of at least one power storage device of said main power storage device and said plurality of sub power storage devices (BB1, BB2) is satisfied.