A power supply device for an auxiliary device battery that is mounted on an electric motor vehicle which travels by using drive power stored in a drive battery, comprising: an auxiliary device battery that stores the power for an auxiliary device of the electric motor vehicle, characterized by further comprising: a converter that works by using external power which is supplied from an external power source through a charging cable which is connected to a charging port of the electric motor vehicle, and supplies the power to the auxiliary device battery by converting the external power into the power for the auxiliary device; an opening and closing switch that is arranged in a circuit which supplies the external power to the converter from the charging cable, and turns on or turns off a connection of the circuit based on an operation of a user; a control unit that works by using the power for the auxiliary device which is stored in the auxiliary device battery, and carries out a control process of the electric motor vehicle; and a start switch that accepts a start operation of the control unit by the user, wherein when both of the opening and closing switch and the start switch are in an ON state, and a stored power amount of the auxiliary device battery is less than a startable power amount of the electric motor vehicle, the converter supplies the external power to the auxiliary device battery, and makes the stored power amount of the auxiliary device battery be equal to or more than the startable power amount of the electric motor vehicle so that the control unit is started. an auxiliary device battery that stores the power for an auxiliary device of the electric motor vehicle, characterized by further comprising: a converter that works by using external power which is supplied from an external power source through a charging cable which is connected to a charging port of the electric motor vehicle, and supplies the power to the auxiliary device battery by converting the external power into the power for the auxiliary device; an opening and closing switch that is arranged in a circuit which supplies the external power to the converter from the charging cable, and turns on or turns off a connection of the circuit based on an operation of a user; a control unit that works by using the power for the auxiliary device which is stored in the auxiliary device battery, and carries out a control process of the electric motor vehicle; and a start switch that accepts a start operation of the control unit by the user, a converter that works by using external power which is supplied from an external power source through a charging cable which is connected to a charging port of the electric motor vehicle, and supplies the power to the auxiliary device battery by converting the external power into the power for the auxiliary device; an opening and closing switch that is arranged in a circuit which supplies the external power to the converter from the charging cable, and turns on or turns off a connection of the circuit based on an operation of a user; a control unit that works by using the power for the auxiliary device which is stored in the auxiliary device battery, and carries out a control process of the electric motor vehicle; and a start switch that accepts a start operation of the control unit by the user, wherein when both of the opening and closing switch and the start switch are in an ON state, and a stored power amount of the auxiliary device battery is less than a startable power amount of the electric motor vehicle, the converter supplies the external power to the auxiliary device battery, and makes the stored power amount of the auxiliary device battery be equal to or more than the startable power amount of the electric motor vehicle so that the control unit is started.