A power source system for an electric powered vehicle equipped with a motor generating vehicle driving power, comprising: a main power storage device for storing electric power input to and output from said motor; a sub power storage device having an output voltage lower than an output voltage of said main power storage device; a voltage converter configured to convert the output voltage of said main power storage device to a level of the output voltage of said sub power storage device, and output the converted output voltage to said sub power storage device; and a first controller operated using electric power from said sub power storage device, for monitoring states of charge of said main power storage device and said sub power storage device and controlling operation and stop of said voltage converter, said first controller being configured such that, in a vehicle running state, said first controller is always operated, and always operates said voltage converter to maintain the output voltage of said sub power storage device at a target voltage, and when said electric powered vehicle is in a key-off state, said first controller is intermittently activated from a sleep mode to an operating mode in response to an activation trigger, and if the output voltage of said sub power storage device becomes lower than a prescribed voltage during operation, said first controller performs charging processing for said sub power storage device using the electric power of said main power storage device, by operating said voltage converter, characterized in that it comprises a charging connector for establishing electrical contact with an external power source external to the vehicle; a charger for converting electric power from said external power source supplied to said charging connector, into charging power for said main power storage device; a charging relay (CHR1, CHR2) for controlling connection and cut-off between said charger and said main power storage device; a second controller operated using the electric power from said sub power storage device, for controlling said charger to charge said main power storage device by said external power source when a prescribed condition for external charging is satisfied and thereby transition from said key-off state to an external charging state is made; a main relay (SMR1, SMR2) controlling connection and cut-off between said main power storage device and a main power supply line; a power control unit configured to control driving of said motor by power conversion between said main power supply line and said motor during said vehicle running state; and a third controller (80, 81) operated using the electric power from said sub power storage device, for controlling said power control unit to drive said motor in accordance with a running state during said vehicle running state, wherein, in said key-off state, said main relay (SMR1, SMR2) and said charging relay (CHR1, CHR2) are opened, and said second controller, said third controller (80, 81) receive power supply from said sub power storage device only for activation processing, and said charger, and said power control unit are stopped, wherein said first controller is configured such that, if the output voltage (Vs) of said sub power storage device is higher than said prescribed voltage during intermittent operation in said key-off state, said first controller determines a degree of deterioration of said sub power storage device based on changes in said output voltage detected at each said intermittent operation, and said first controller is configured such that, during the intermittent operation in said key-off state, said first controller sets a cycle of the intermittent operation based on the determined degree of deterioration of said sub power storage device. a main power storage device for storing electric power input to and output from said motor; a sub power storage device having an output voltage lower than an output voltage of said main power storage device; a voltage converter configured to convert the output voltage of said main power storage device to a level of the output voltage of said sub power storage device, and output the converted output voltage to said sub power storage device; and a first controller operated using electric power from said sub power storage device, for monitoring states of charge of said main power storage device and said sub power storage device and controlling operation and stop of said voltage converter,