Provided is a power supply device (100) which includes a plurality of battery modules (102) and in which the battery modules (102) are connected in series with one another in accordance with a gate driving signal from a controller (104). The power supply device (100) includes a disconnecting part (22) configured to forcibly isolate the battery module (102) from a series connection regardless of the gate driving signal, and limits, in accordance with a target output voltage value, a number (Npass) of the battery modules (102) to be forcibly isolated by the disconnecting part (22).