A power interchange system preferentially transmits electric power from a suitable power supply. A controller (100) includes signal sending/receiving means (300) for receiving information regarding a power transmitting/receiving device (101); power transmitting/receiving device information storage means (310); priority information storage means (310); and information control means (301) for determining, based on the information regarding the power transmitting/receiving device (101) and the priority of the power supply (104b), an amount of power to be transmitted and the power transmitting/receiving device (101) which is to transmit electric power, and creating, based on the determination, a control instruction to make the power transmitting/receiving device (101) transmit electric power. The signal sending/receiving means (300) transmits the control instruction to at least the power transmitting/receiving device (101), the power transmitting/receiving device (101) transmits electric power according to the control instruction.