An electric power supply device using an electric vehicle, comprising: a drive battery installed in the electric vehicle, configured to supply electric power charged therein to an electric motor for driving the electric vehicle and also to outside of the electric vehicle; an externally connected unit (31, 32) arranged external to the electric vehicle, configured to convert the direct current electric power stored in the drive battery to alternating current power and to supply the converted electric power to an electrical device located outside of the electric vehicle; an electric power on/off unit configured to switch on/off electric power supplied from the drive battery to the externally connected unit (31, 32); an operation control unit (17, 20) configured to control operation of the electric power on/off unit; and a state detection unit (17, 20) configured to detect a state of the drive battery when supplying the electric power is shut off by the electric power on/off unit; characterized in that the operation control unit (17, 20) is configured to cause the electric power on/off unit to shut off supplying the electric power from the drive battery to the externally connected unit, when a predetermined time has elapsed after start of supplying the electric power from the drive battery to the externally connected unit; the state detection unit (17, 20) is configured to calculate a remaining capacity of the drive battery based on an integrated value of the current supplied from the drive battery to the externally connected unit (31, 32) when the electric power is supplied from the drive battery to the externally connected unit (31, 32); the state detection unit (17, 20) includes a voltage detection unit that is configured to detect an open circuit voltage of the drive battery; the state detection unit (17, 20) is configured to calculate the remaining capacity of the drive battery based on a result of detection by the voltage detection unit when supplying the electric power is shut off by the electric power on/off unit; the state detection unit (17, 20) is configured to execute a failure diagnosis for detecting a failure in the drive battery and a failure in the electric power on/off unit when supplying the electric power is shut off by the electric power on/off unit; when no failure both of the drive battery and of the electric power on/off unit is detected, the remaining battery capacity calculated based on a result of detection by the voltage detection unit is memorized replacing the remaining battery capacity calculated by integrating the current value, the operation control unit (17, 20) is configured to cause the electric power on/off unit to restart supplying the electric power from the drive battery to the externally connected unit (31, 32). a drive battery installed in the electric vehicle, configured to supply electric power charged therein to an electric motor for driving the electric vehicle and also to outside of the electric vehicle; an externally connected unit (31, 32) arranged external to the electric vehicle, configured to convert the direct current electric power stored in the drive battery to alternating current power and to supply the converted electric power to an electrical device located outside of the electric vehicle; an electric power on/off unit configured to switch on/off electric power supplied from the drive battery to the externally connected unit (31, 32); an operation control unit (17, 20) configured to control operation of the electric power on/off unit; and a state detection unit (17, 20) configured to detect a state of the drive battery when supplying the electric power is shut off by the electric power on/off unit; characterized in that the operation control unit (17, 20) is configured to cause the electric power on/off unit to shut off supplying the electric power from the drive battery to the externally connected unit, when a predetermined time has elapsed after start of supplying the electric power from the drive battery to the externally connected unit; the state detection unit (17, 20) is configured to calculate a remaining capacity of the drive battery based on an integrated value of the current supplied from the drive battery to the externally connected unit (31, 32) when the electric power is supplied from the drive battery to the externally connected unit (31, 32); the state detection unit (17, 20) includes a voltage detection unit that is configured to detect an open circuit voltage of the drive battery; the state detection unit (17, 20) is configured to calculate the remaining capacity of the drive battery based on a result of detection by the voltage detection unit when supplying the electric power is shut off by the electric power on/off unit; the state detection unit (17, 20) is configured to execute a failure diagnosis for detecting a failure in the drive battery and a failure in the electric power on/off unit when supplying the electric power is shut off by the electric power on/off unit; when no failure both of the drive battery and of the electric power on/off unit is detected, the remaining battery capacity calculated based on a result of detection by the voltage detection unit is memorized replacing the remaining battery capacity calculated by integrating the current value, the operation control unit (17, 20) is configured to cause the electric power on/off unit to restart supplying the electric power from the drive battery to the externally connected unit (31, 32).