A power supply unit includes a communication unit and a control unit. The communication unit is capable of performing communication with a first processing unit group constituted of a plurality of processing units connected thereto. The control unit controls powers to the plurality of processing units through the communication so that the powers are turned on in an order corresponding to an order of connection and assigns, to the plurality of processing units, respectively, IDs of numbers corresponding to the order of turning-on of the powers each time the power is turned on.