A power supply device includes a plurality of power supply units connected in parallel, and includes a positive electrode-side junction formed by the joining of positive electrode-side terminals of the plurality of power supply units and a negative electrode-side junction formed by the joining of negative electrode-side terminals of the plurality of power supply units. Each of the plurality of power supply units includes a battery unit, and a first relay that is provided between one of the positive and negative electrode-side junctions and the battery unit and is electrically disconnectably connected in series to the battery unit. Each of the plurality of power supply units includes a resistive element of which one end is connected between the first relay, which is connected to one of a positive electrode terminal and a negative electrode terminal of the battery unit, and the battery unit.