A power supply device comprising: a carriage having wheels ; an inverter supported by the carriage ; a battery having a power output terminal , the power output terminal being located at a lower part of the battery ; and a battery mounting portion to which the battery can be mounted and detached by moving vertically, wherein the battery mounting portion has a power input terminal connected to the inverter and contact with the output terminal when the battery is mounted, and characterized by a load monitor having: a first switch provided between an outlet connected to a load and the inverter ; a DC power supply connected to the outlet in parallel with the inverter ; a second switch provided between the DC power supply and the outlet ; a first sensor detecting a current flowing between the outlet and the inverter ; a second sensor detecting a current flowing between the outlet and the DC power supply ; and a controller configured to make the first switch OFF, the second switch ON and stop an operation of the inverter when the current detected by the first sensor is smaller than a first threshold for a predetermined period, and make the first switch ON, the second switch OFF and start an operation of the inverter when the current detected by the second sensor exceeds a second threshold. a carriage having wheels ; an inverter supported by the carriage ; a battery having a power output terminal , the power output terminal being located at a lower part of the battery ; and a battery mounting portion to which the battery can be mounted and detached by moving vertically, wherein the battery mounting portion has a power input terminal connected to the inverter and contact with the output terminal when the battery is mounted, and characterized by a load monitor having: a first switch provided between an outlet connected to a load and the inverter ; a DC power supply connected to the outlet in parallel with the inverter ; a second switch provided between the DC power supply and the outlet ; a first sensor detecting a current flowing between the outlet and the inverter ; a second sensor detecting a current flowing between the outlet and the DC power supply ; and a controller configured to make the first switch OFF, the second switch ON and stop an operation of the inverter when the current detected by the first sensor is smaller than a first threshold for a predetermined period, and make the first switch ON, the second switch OFF and start an operation of the inverter when the current detected by the second sensor exceeds a second threshold. a first switch provided between an outlet connected to a load and the inverter ; a DC power supply connected to the outlet in parallel with the inverter ; a second switch provided between the DC power supply and the outlet ; a first sensor detecting a current flowing between the outlet and the inverter ; a second sensor detecting a current flowing between the outlet and the DC power supply ; and a controller configured to make the first switch OFF, the second switch ON and stop an operation of the inverter when the current detected by the first sensor is smaller than a first threshold for a predetermined period, and make the first switch ON, the second switch OFF and start an operation of the inverter when the current detected by the second sensor exceeds a second threshold. <EOS>